inzendium

Hero of Leina Tower could also be a good Heroic creature to add.

First thing: what are you trying to accomplish in this?(not trying to criticize) If you are going to focus on Evolve, you might want to pull out some non-evolve creatures (e.g. Slaughterhorn, Deepwater Hypnotist, etc.)
Since I have no clue what your budget would be, I am just going to give suggestions and try to keep it low. And remember, whether you make these changes is completely up to you.
What I would do:

-3 Swordwise Centaur, +3 Renegade Krasis
-3 Staunch-Hearted Warrior, +3 Centaur Battlemaster
-2 Deadly Recluse, -1 Deepwater Hypnotist, +3 Vorel of the Hull Clade

Also, you probably need to add more lands. 17 lands (I guess 18 in a way, because of Simic Growth Chamber) is running a bit low for any deck. I would drop Slaughterhorn, Kessig Recluse, Netcaster Spider, the Omenspeakers, and the Beetleform Mages to add in more lands, like Thornwood Falls or just plain Islands and Forests. Again, this is entirely up to you, and you can choose to drop different cards.

Lastly, try to balance out your mana base. Rather than making it 50/50 of blue and green mana, try to make a ratio equal to the ratio of green and blue cards in your deck. The "Mana Symbol Occurrence" shown on the deck's info on the right helps you see the ratios without having to go through and count each card. For example, you are going to want to have more Forests than Islands in this deck, but how many more is going to be up to you.

I hope I helped. I like to help newer players along. To be honest, I have also been playing for about a year, but I have gotten in a lot of playtime and have seen a vast variety of cards. I'll be happy to offer more suggestions if I think of them :D

As for X cost spells, Gelatinous Genesis or Exsanguinate would be good.

No Nicol Bolas, Planeswalker? His abilities are just too overpowered. Also, since budget seems to have no say, why not an Ugin, Spirit Dragon? He would help throw out all your other planeswalkers.

While Blightsteel Colossus would be tons of fun to play, it's just too expensive for me. As for Core Prowler, I decided this deck would be too fast paced for me to want to add in proliferation. I would have added Contagion Clasp or Contagion Engine if I was. Thanks for the ideas, though. :D

Suggestions:

-Greater Auramancy
-Privileged Position
-Zur the Enchanter (for grabbing choice enchantments)
-Energy Chamber (to boost Astral Cornucopia and Aether Vials; you can toss out the AC for free and give it charge counters every turn)
-Plea for Guidance/Idyllic Tutor/Enlightened Tutor
-Serra's Sanctum

Looks like fun to play! If only the good cards weren't so expensive xD

Is Pyromancer's Gauntlet too high of a cmc for this? It might help if you don't get Hidugetsu's Second Rite, or help speed up the game if you aren't getting the right burn cards and need to hurry and finish it. Sideboard?

With discard, Waste Not and Liliana's Caress will certainly be useful. Maybe some Mind Sludge would also help you out, since in a mono black deck that is a hand-dropper. You might want to go a little heavier on lands, but other than that it looks pretty good.

Energy Chamber? Low mana cost, and will help boost creatures or Banshee's Blade if that's out.

Wardscale Dragon would fit nicely into here. Xenagos, the Reveler would help you throw out some big guys, as well. Lastly, you might be running too few lands, but that's just my opinion. You might consider swapping cards you don't need as much for more lands, since, as you said, this is more of a late-game deck. Other than that, looks good!

Nothing that I really need feedback on at the moment. Thank you for the offer, though
Also, maybe Goblin Bombardment or Barrage of Expendables? Sac goblin tokens or unimportant goblins to give small damage and wear them down bit by bit.

Glad I could help! :D

Suggestions (not sure which of these would fit in Modern):

-Obelisk of Urd
-Door of Destinies
-Coat of Arms
-Krenko, Mob Boss
If you do add Krenko:
-Sideboard Illusionist's Bracers or Magewright's Stone

Not sure how many of these will fit the theme you are trying to keep, but here are some suggestions:

-Mind Sludge for heavy discard
-Quag Sickness for bringing down creatures some more
-Nightmare or Squelching Leeches for a finisher
-Swap a couple Doom Blade with a couple Go for the Throat or Ultimate Price for more diversity in removal spells
-Brain Maggot can be good for taking cards away from their hand and can get annoying quick

Black is my favorite color for monocolored decks and this looks pretty good overall.

I would add Virulent Wound or Grim Affliction for taking care of their creatures and giving them another couple counters. Caress of Phyrexia might be a game-finisher in a pinch (sideboard?).

Suggestions:
Nefarox, Overlord of Grixis (more sacrifice along with handy exalted)
Dictate of Erebos (they sacrifice every time you do)
Archfiend of Depravity might also help keep their stuff under control
I hope I gave you some helpful ideas.

If you want to get boosts from their graveyard, I would add Wight of Precinct Six. If you don't care, Leyline of the Void, Grafdigger's Cage, and Crypt Incursion could help you keep them from salvaging cards.

Didn't add Utopia Sprawl simply because it is very expensive. Does Rampant Growth work instead?
The main problems for this deck would be not getting my creatures out or not throwing a damage enchantment out, but I have mostly creatures and eight of the enchantments, so I think I gave a good chance of one. Also, the deck is not reliant on the enchantments, so it can still work even if I don't get one.

I'm going to edit the lands tomorrow.

I didn't realize it was expensive, as I'm not checking the prices on these as I go. Mind sculpt is a decent alternative to it.

Havoc Festival? I don't really know Rakdos that well, so I don't know if that would be a good idea. Also, I don't mean to advertise, but could you tell me what you think of this?
http://www.mtgvault.com/inzendium/decks/sickness-slinger/
Good deck, by the way. +1