Muktol

The new "Toxic" keyword and the new Phryexia expansion in general could also add some interesting cards to the deck:
[[Crawling Chorus]], [[Infested Fleshcutter]], [[Jawbone Duelist]], [[Skrelv's Hive]], [[Skrelv, Defector Mite]], [[White Sun's Twilight]], [[Experimental Augury]], [[Prologue to Phyresis]], [[Basilica Skullbomb]]

Some people have a friend/ fanbase, they get more comments, others... simply need luck.
Normally I give detailed comment only when someone has a proper deck-description, decks without a description get no comments and likes at all or, if I REALLY like the concept/ idea, only some quick card suggestions, without any additional words. Guess you got lucky.

[[Remember the Fallen]], [[Emerge Unscathed]] and [[Contentious Plan]] come to my mind when seeing this deck.

[[Order of the Stars]], [[Door to Nothingness]]

[[Carrion Feeder]], [[Mortician Beetle]], [[Gixian Infiltrator]], [[Hordeling Outburst]]
A deck-description would be a nice addition to your decks :)

I have tested the original list, but not this one.

[[Talon Sliver]] came to my mind because it was mostly using the shapeshifters in defensive situations and first strike should allow them to survive a little bit better. The question was: Add a 4th [[Sinew Sliver]] or something new, and I quickly looked at my sliver-list and found this card. [[Crypt Sliver]] could also be an option in terms of survival tough it doesn't add to the decks combat abilities.

As for the lands: I rarely had the problem of being colour-screwed, and even the situations where I had problems with mana in general where very few. Also I really love the New Capena search-lands (like [[Obscura Storefront]]) and them it instead of [[Evolving Wilds]] in every multi-coloured pauper deck I have. And they are cheaper than [[Ash Barrens]].

@wickeddarkman
I took a look at [[Venomous Changeling]]. Personal feeling is that it would fit into the deck and it wouldn't fit into the deck at the same time. On one hand the deck needs at least 1 Shapeshifter on the battlefield boost [[Griffin Rider]], on the other hand it wants as many Shapeshifters on the field for [[Smitten Swordmaster // Curry Favor]].
[[Venomous Changeling]] is way better than a 1/1 any of the other shapeshifters in this deck. But Deathtouch is an advantage and drawback at the same time because it keeps opponents from attacking but it will draw and removal an opponent has. And with 3 mana its also on the upper scale of the deck.

[[Ghyrson Starn, Kelermorph]]?
A short deck-description would also be a nice addition.

Feel free to do so. I would happy, if you manage to play the deck, if you could give me some feedback on how it worked out or what you changed :)

In my tests I got the following results:
Boros land destruction 5-2
Dimir defensive mill 6-1
Mono black control 4-3
Mono green stompy 3-4
Mono red Red reck wins 3-4
Orzhov life gain/Drain 4-3
Selesnya tokens 3-4

If you want to print some proxies to give cards a try I can recommend https://www.mtg-print.com/. The got good quality images of the cards (in English) and even have some other language-version of the cards available (Although not in the same image quality). I know some people vote against proxies, but I think they're a great way of trying out different cards, before you actually buy them.

Only the Double Master version of Splinterfright seems to be bugged, Innistrad and Commander seems to work.

[[Winding Way]] or [[Wreath of Geists]] could fit in here.

As I really like the idea, I took the time to test this against my pauper benchmark decks. Perhaps this will help you tuning the deck and adapting it to your likes.
I haven't used the sideboard, only did 7 straight games.

VS. Boros Land destruction: 4-3
[[Goldmire Bridge]] saved my ass in 2 of the 7 games, because otherwise I would have no mana to cast anything. The only other thing that can be a problem in Boros LD would be [[Guardian of the Guildpact]].

VS. Dimir Mill: 2-5
[[Pestilence]] is a huge problem as it obliterates this deck, tough boarding [[Disenchant]] would change the chances drastically. [[Sinew Sliver]] can help a little bit here, but you need some of them on the battlefield to make a difference. [[Crypt Incursion]] is another thing to fear.

VS mono Black Control (MBC): 1-6
My MBC also includes [[Pestilence]], but even after sideboarding I fear that this deck wouldn't stand a good ground against it, as MBC simply has way to much other removal. [[Cuombajj Witches]] are another huge problem, simply pinging away all those small shapeshifters.

VS mono green stompy: 3-4
Life-gain and [[Griffin Rider]] are the key here to stay alive and keep Stompy under control... if it doesn't stomp you into the ground in an instant. [[Hunger of the Howlpack]] and [[Rancor]] can quickly turn a flimsy [[Nettle Sentinel]] into a real beast, not to speak of a live-giving [[Vault Skirge]]. At this point I would really like to have some removal into the deck (e.g. [[Cast Down]]), to deal with dangerous creatures my own creatures aren't able to handle (To big, or evasion)

VS mono red red deck wins (RDW): 4-3
Against RDW you have to survive the first turns, trade your own creatures against attackers and basically wait (and survive) till your opponent runs out of ammo... hopefully he does before you run put of life. When your opponent is in topdeck-mode the game is basically over for him, because you can start gaining life and making a vicutory impossible for him.

VS Orzhov Life gain/ drain: 1-6
"YEAH, I've manage to deal 10 damage to my opponent....never mind your life-points are back where they where the turn before." This is what I would summon the games against this deck. An opponent simply sits there, waiting for either [[Marauding Blight-Priest]] and watches you explode without attacking, or for [[battle Screech]] and overruns you with these flyers. Sideboarding [[Suffocating Fumes]] or [[Arms of Hadar]] would turn the tides. The only game I was able to win was when I got 2 [[Griffin Rider]] on the battlefield and was able to hammer my opponent's life down slowly.

VS Selesnya tokens: 1-6
This is a damage race, but one you will only rarely win. [[Crusader of Odric]] and [[Scion of the Wild]] will grow stronger and stronger while you can only sit and watch. And if your opponent manages to draw and play the endless token combo, the only thing you can do is put your cards down and say "Thank you for the game".

Conclusions:
Personally I would try to fit in some removal into the deck, 4 [[Cast Down]] or even 3 of those and 1 [[Suffocating Fumes]]. I also would remove [[Deadly Dispute]], or at least switch it for [[Village Rites]]. In all the games I was only able to use the treasure token 2 or three times, once for mana and the other two times to cast a second [[Deadly Dispute]]. Moe often I was missing the second mana to cast it in this situation, or didn't have a good target to sacrifice. Another thing I noticed during play is the high number of lands that enter tapped, which can slow you down quite a bit. [[Goldmire Bridge]] was good against land destruction, but else slowed me down more than it helped to fix mana.
Most of the time I had 2-4 shapeshifters on the battlefield, so [[Rat Colony]] might not be as good as I thought it might be.

The "Ally" or "Party" mechanic comes to my mind when thinking of shapeshifters. 22 lands seem to be on the high-side, for an aggressive deck like this one, personally I would try to lower it to 20, which would give you 2 free slots to add cards to fiddle around with.

Another black card that would fit into the deck would be [[Rat Colony]]. If you want to push the deck more into the control-direction you could use [[Spellstutter Sprite]] and [[Aquitect's Will]], tough that would require the use of blue mana.

No [[Ethereal Armor]]? [[Helm of the Gods]] could also be worth a look as [[Champion of the Flame]] benefits from enchantments and equipments... and its colourless so no problem with the mana.
24 lands seems a little bit to much here, I would probably go down to 20 lands.

[[Quirion Dryad]] could be replaced with [[Managorger Hydra]] or [[Quandrix Pledgemage]]. The Hydra has the advantage of getting a +1/+1 counter whenever a spell is cast, even when your opponent does so, while Pledgemage simply allows you to benefit also from your mono green spells. Drawback is that both creatures costs {1} more mana to cast.

What about storm cards? [[Chatterstorm]] or [[Weather the Storm]] aren't that expensive and [[Dragonsguard Elite]] (or [[Quandrix Pledgemage]] if you add him) could also profit from the copies.

[[Mulch]] and [[Spider Spawning]] where a common tactic during Innistrad. [[Wreath of Geists]] and the "Undergrowth" mechanic (e.g. [[Molderhulk]]) could also be worth a look, tough you might need more creatures in the deck for this.

[[Cackling Counterpart]] normally is used to get a creature not only 4 times on the battlefield, but even more often, but you don't have any creature 4 times in your deck, so what do you want to achieve with this card?

[[Archway Commons]] some way slipped 5 times in the deck, 4 is the maximum number.

When it came to gaming sessions with friends it was always a fun one to see my wife's Krenko deck clash with my friends werewolf deck. Who can deal more damage faster ;)
And yes, it was like you said, Krenko and Haste is really a gamechanger. I've seen matches where you would attack on T3 with [[Warren Instigator]] and put, because the opponent had no defence, 1 [[Goblin Chieftain]] and 1 [[Krenko, Mob Boss]] on the field and instantly tap Krenko for at least 3 Tokens. In her Krenko commander deck we used untap-artifacts, like [[Thornbite Staff]] to make things even worse for the opponents.

Here Is what I would do personally, tough I have to say that I haven't played modern in a loooong time.

Max out the number of [[Goblin Chieftain]] due to its haste ability. Krenko without haste is a sitting duck, that just cries "Please remove me". With haste your opponent can't prevent you from activating his ability once he is on the battlefield.
Remove [[Skirk Prospector]], [[Chandra, Torch of Defiance]], [[Squee, Dubious Monarch]] and [[Muxus, Goblin Grandee]]. Whule you can get 1-2 mana out of the Prospector the question is... which one of your precious goblins are you willing to sacrifice? Every one plays a part in your strategy and normally you don't want to loose any of them. After you get Krenko on the field you can safely start sacrificing tokens... but what for? To overkill an opponent?

Sone general suggestions, based on my wife's Krenko deck, which she has played plenty of times:
[[Goblin Bushwhacker]], [[Goblin Piledriver]], [[Warren Instigator]]
[[Goblin Grenade]], [[Quest for the Goblin Lord]], [[Shared Animosity]]

I'm using [[Hands of Binding]] in one of my own decks but I've titled it "Unblockable" which gives me a little more freedom in the choice of the spells I'm using. Against slow decks this strategy worked dine, but against very aggressive or very control-heavy opponents the deck had problems standing its ground.

Nevertheless, some suggestions:
[[Changeling Outcast]], [[Tormented Soul]], [[Slither Blade]], [[cMiscreant]]... Cheap creatures, the first 3 have the unblockable ability, while [[Faerie Miscreant]] "only" has flying, but can give you some additional card advantage.
[[Dismal Backwater]], [[Obscura Storefront]]... additional/ alternative mana fixing, if needed.
[[Morsel Theft]], [[Sigil of Sleep]]... This are the spells I'm using for my unblockable creatures, to keep my opponent under control. [[Morsel Theft]] isn't reusable, but cheaper than [[Shadow Slice]] and it also provides you with some life.

[[Kaijin of the Vanishing Touch]]/ [[Wall of Tears]] are creatures that help to protect you and can cost and opponent some time... and nerves.
[[Deep-Sea Kraken]] would be a BBB, and would fit into the decks theme.

[[Obscura Storefront]] could search for all 3 of the lands you need and solve some of those problems.

2 Rectangular brackets before and after the card name: [[CARDNAME]]. Tough this only works in comments, not on descriptions or the forum, and cards with an apostrophe in its name, like [[Chainer's Edict]], also don't work.

I have tested a U/B bounce deck that uses [[Cavern Harpy]] and [[Dream Stalker]] to get things back, tough the version plays more like a control-ish Boros bounce/ monarch.

[[Cauldron Familiar]] could negate the life-loss from [[Cavern Harpy]] and (very) slowly drain an opponent. [[Chittering Rats]] can wreck an opponents draw. [[Radiant Fountain]] is life-gain and can be flickered with [[Ghostly Flicker]], tough you might ave better targets for the spell.