Muktol

When using monoG, and especially monoG Devotion there are some nice cards to consider:
Khalni Hydra ...costing massive 8 green mana for a 8/8 with trample. The nice thing about her is that the more green creatures you have on the field the less this giant costs to cast.
Leatherback Baloth or Wilt-Leaf Cavaliers ... 3 green mana for a 4/5 or a 3/4 with vigilance. A good creature that can be used very versatile and brings 3 devotion on the field.
Primalcrux ...6 green mana for a */* (At least a 6/6). Its strength and toughness is defined via devition.
Utopia Sprawl or Overgrowth ... if you're not up for the lifegain then those are better choices than Verdant Haven. Also Arbor Elf might be better than Elvish Mystic when working with land-enchantments.

Greetings
Muktol

Basically it seems that you're trying to build an elf-deck with some additional finishers. For this I would put some cards into the sideboard and add some others to make it more efficient. So here are the changes I would do:
Gaea's Herald... sideboard. Not every match you will play will have counterspells involved.
Reclamation Sage ...sideboard. Again, not every match you play will have artifacts or enchantments that need to be removed involved.
Predator's Rapport ...sideboard at least 1-2 of them and use against decks that really put you under pressure.
Drove of Elves ...better choice than Jagged-Scar Archers as it has a built-in hexproof and thus increased chance of survival.
Arbor Elf ... better choice than Elvish Mystic as it allows you to untap forest enchanted with Overgrowth or Utopia Sprawl

Suggestions:
Imperious Perfect ...+1/+1 for all other elves and you can produce a 1/1 elf token every turn
Wren's Run Vanquisher ... cc2 3/3 and deathtouch, an elf nobody wants to fight against.
Dwynen, Gilt-Leaf Daen ......+1/+1 for all other elves and when he attacks you get 1 life for each attacking elf.
Elvish Visionary ... cc2 1/1, thus a cheap blocker but he allows you to draw a card when he enters the battlefield.
Ranger's Guile or Blossoming Defense ...helps you to protect your creatures against point-removal

Greetings
Muktol

I would nevertheless consider using blue and Paradox Haze here. There are enough interesting cards with ETB-triggers in blue to find a replacement for Primordial Hydra. It would also give you access to Etherwrought Page which is a very versatile card.

As you are running mostly elves here Elvish Archdruid would be an option. It would give all your (other) creatures +1/+1 and allow you to produce a huge amount of mana. Personally I would get rid of Boggart Ram-Gang for it.
Joraga Warcaller is also a card to consider (like you mentioned). Depending on how well you feel with your manabase you could try to change Elvish Mystic against the Warcaller.
As you will run out of cards fast you could also consider some carddraw like Lead the Stampede (Or the more expensive ($) but more effective Collected Company). Only question would be what to remove for it.
As for the sideboard...the question is what you feel that you need, what decks you think you will be weak against. This depends a lot on the environment you're playing in and the best way to get a feel for this is simply asking other players for help after a tournament.

Greetings
Muktol

Docent of Perfection could be an interesting card here as well as Baral, Chief of Compliance (Battlefield Thaumaturge would be the more budget friendly version).
The main problem with Coat of Arms is that it count for each creature on the field, yours and your opponents. So if you're e.g. fighting an elf/ goblin/ human/ any other tribal deck and can't counter enough creature spells you might easily dig your own grave. A safer approach would be Dragon Throne of Tarkir or Junkyo Bell.

Greetings
Muktol

Here's a basic tutorial on deck-building that I did some time ago.
http://www.mtgvault.com/muktol/decks/how-to-build-a-deck/
This might help you with some of the basic decisions that have to be made on how your deck should evolve. When you need help some more informations would also be nice: Are you willing to buy/ trade cards, maximum budget for this, perhaps the desired legality of the deck, and so on. Basically every information you can give us beforehand we don't have to ask afterwards :)

Greetings
Muktol

It seems that you misunderstood something in the pauper rules (or stumbled upon one of the false tagged/ named decks here on the site) as a pauper deck consists only of common cards. So no Duskwatch Recruiter & co.
If you want to mark this as a cheap deck the word budget would be better.
Also a deck and how-to-play description is missing.

Greetings
Muktol

Sorry that the answer took so long, I've been on holiday ;)
There are several cards that could be used when using blue, tough the number of cheap and fast ones is limited: Grixis Grimblade, Prized Amalgam (Works like a charm with Geralf's Messenger or Gravecrawler), Skaab Ruinator (May be a little bit hard to cast) or Stitched Drake (Flying). Havengul Runebinder could also be of interest tough you have to exile a creature to pay his effect.
Undead Alchemist would enable you to switch your deck from kill to mill, tough I'm not a real big fan of that card as it basically doubles/ triples the damage you have to deal to your opponent.
On the support side you could use control spells like Rune Snag or evasion effects like Artful Dodge.

+1
Undead Warchief and Gray Merchant of Asphodel could be some ideas. The warchief is a very aggressive drop, giving all your zombies, including himself, +2/+1 and lowering their casting-costs by {1}. Gray Merchant of Asphodel can finish of your opponent later on very quickly without having to attack.
Splashing blue would give you access to Havengul Runebinder and Diregraf Captain. The runebinder can convert your dead creatures into 2/2 tokens and +1/+1 counters for everyone while Diregraf Captain is another captain that makes your opponent lose life whenever you lose one of your zombies.

Greetings
Muktol

The one reason that would come to my mind why to keep it is rather simple. Disturb your opponents game-plan. While not as strong as discard or removal it helps by moving cards your opponent might need somewhere it will be hard for him to get to: The graveyard.

I would try to streamline this deck a little bit. Remove/ sideboard some creatures to make space for other spells needed.
Bloodline Keeper and Vampire Nocturnus might be too much in a deck as both have casting costs of 4. I would up the number of Nocturnus to 4 and lower the number of the Keeper by 1-2.
Remove Falkenrath Exterminator as he is simply to weak without support. Simple removal spells might be better here (See suggestions). Kalitas, Bloodchief of Ghet is too expensive.
I would sideboard 2-3 Vampire Hexmage. It's a good vamp but it's real strength is removing planeswalker and other creatures that use counters, and you simply don't run into those every match.
If you want the effect from Necropolis Regent but cheaper (Thus faster) you can try Curse of Stalked Prey or Rakish Heir. The curse is hasrder to remove but the Heir gives your another body that could attack or defend.
Personally I don't like Falkenrath Marauders. Yes they can become strong but there are better creatures in the cc5 slot and they can be removed by a simple Shock.

Some further suggestions:
Dreadbore, Terminate, Go for the Throat, Doom Blade, Lightning Bolt, Dismember, Tragic Slip ... simple but effective removal that will allow your vampires to get to your opponent. If you want something in creature-form Guul Draz Assassin or Gatekeeper of Malakir might be worth a look.
Blade of the Bloodchief ... whenever a creatures dies the equipped creature gets 2 +1/+1 counters.
Read the Bones, Sign in Blood ... carddraw that provides you with additional ammunition against your opponent.
Blackcleave Cliffs, Dragonskull Summit ... some good, fast and relatively cheap duallands. There are others but those are more expensive or have the disadvantage of entering tapped.
Evil Presence might be an interesting sideboard-option as it can ruin your opponents day when he really needs a special land.

Greetings
Muktol

Well you can (lategame) most of the time bounce a tapped land and thus play this 2/3 fyling for a single {W}, but yeah, you don't have any cards with a special ETB trigger in this deck and providing flying to an additional creature normally will catch your opponent off-balance.

Personally I would switch Kor Outfitter for something like Kor Aeronaut or Kor Skyfisher as something with evasion will bring you more than a free equip ability.

Greetings
Muktol

Many of the decks used Cloudshift/ Ghostly Flicker to re-use Gatecreeper Vine.
Expedition Map could also be used to search Maze's End out of your deck which then can search other gates.

For untapping lands with a planeswalker I would suggest Garruk Wildspeaker, tough he is a little more expensive ($) than Kiora but he can untap two lands per turn. Another (not really budget friendly) version of getting your gates out would be Primeval Titan as he can search for any land, not only basic lands. Expedition Map could also help you to get the last Gates out on the field (and is nearly budget against the other two suggestions).
The other basic though would be to use only 2-3 colours of mana to give it a more stable play. Tough you have 10 gates getting the right one in your hand to play spells like Kiora and a Doom Blade afterwards might be a problem. Maze's End decks saw action in some standard decks but I fear that those decks are much slower than modern legal ones.

Greetings
Muktol

+1, tough the description(s) could be a little more detailed ;)
Seal of Doom (or Sinister Concoction if you don't mind the mill and self-discard) would be an option instead of the 2 Doom Blade. Costing 1 mana more to play but may increase your devotion by an additional point. Sinister Concoction would be another option, boosting for a massive 3 devotion but using it's ability would have a hefty price.

Greetings
Muktol

Thank you Palaios for the explanation. I never thought of targeting the PW with the ability to reset it.

2 Felidar Guardian in combination with Decoction Module will work giving you any amount of energy (According to the rules, if it's not an infinite loop, a value must be said). Your opponent can nevertheless interrupt this with a simple 'destroy target creature' spell, targeting one of the two Felidar Guardian's.

Felidar Guardian and Saheeli Rai's -2 ability wont work because you're generating a token. The tokens comes into play, triggering it's ability, you exile the original Felidar Guardian and bring it back to the field. The original triggers it's ability exiling the token Felidar Guardian...which is removed from the game as state based action (SBA) as tokens can only function on the battlefield, but nowhere else.

The error in reasoning you had with the Whirler Virtuoso + Decoction Module + Panharmonicon combo is that Whirler Virtuoso's ability is no triggered ability but an activated ability thus not copied by Panharmonicon. Triggered ability's have a "When" "Whenever" "At" in it's wording while activated ability's have a "Cost : Effect". Planeswalker ability's count as activated ability's where adding/ removing loyality counters is the cost and the rest is the effect.
Whirler entering produces 8 energy
-3 energy to produce a token (5 energy) and get 2 energy back when the tokens enters (7 energy)
-3 energy to produce a token (4 energy) and get 2 energy back when the tokens enters (6 energy)
-3 energy to produce a token (3 energy) and get 2 energy back when the tokens enters (5 energy)
-3 energy to produce a token (2 energy) and get 2 energy back when the tokens enters (4 energy)
-3 energy to produce a token (1 energy) and get 2 energy back when the tokens enters (3 energy)
-3 energy to produce a token (0 energy) and get 2 energy back when the tokens enters (2 energy)
Totally you get 1 Whirler Virtuoso and 6 tokens. Not a bad combo for standard but far from infinite.

Greetings
Muktol

In most other cases I would say yes, especially if it would be another cc1 spell that could help to cast Talara's Battalion. In this special case I wouldn't add it (at least not mainboard) because kicker brings a complete new mechanic into a beginners deck which you would have to explain. When I see that the player starts to get used to playing magic then it can be traded for titanic Growth, I totally agree on that.

I know exactly what you're meaning as this was also one of my concerns, but in the end I wanted to give it a try because of it's incredible good power/toughnes/ mana ratio and trample ability. First I had 4 Battalion in mind but decided against it because of the restrictions when casting it. In the end I lowered it's number to 3 and removed 2 Titanic Growth to add 2 Giant Growth to have a cc1 green spell in the deck to make it easier to cast.
Kalonian Tusker was on my list but I decided against him because of the lack of trample. Strangleroot Geist is a good creature but would bring another mechanic, Undying, with him, something I wanted to avoid for a single card-choice. I found that the good, but simple, cc2/cc3 trample creatures are a rather rare catch.
Groundbreaker would be a strong option but I already have Spark Elemental and loosing the alternative working creature isn't in my likings. Hungry Spriggan could also be an option.
For the moment I would add 3 Hungry Spriggan to the sideboard and see how the deck behaves on some testgames. Next week there will be a small tournament with friends where, when choosen, it will be tested. As I'm breaking in the new players I should be able to see if this will work rather quickly and make a change to the deck within minutes.