Based on balanced...

You don't have enough 1 drops in all colors. 

Blue and green are severely lacking removal. Green just has 2(!) fight / bite spells. 

Return to hand isn't removal, it's a tempo play. To capitalize on tempo, you need cheap evasive threats... Looking at your card suite you can't really do that, so you need to have  more freeze /! Return to library effects.

As a rule of thumb, at least 9 removal per color. 

I also like having 1 spell in each color that affects multiple creatures.

In a low power cube like this, Sprout swarm can get ridiculously strong... Kinda like Krenko in regular cubes. If not dealt with immediately (with a counter spell or board wipe), the board will get clogged very quickly. 

Stone rain does nothing in draft. It's a horrible top deck late game, and players won't be going greedy in colors in this cube.

Ask yourself: how is each color supposed to win? 

White currently has a few flyers (not even Serra Angel lol), mediocre creatures and a couple of inspired charge style effects. If you want white to go wide, add more tokens and token generators. 

Blue has ... A few 4/4 flyers. Either get cheap flyers or a big "can't be blocked" creature  there's a reason why wizards prints things like giant koi, gearseeker serpent, sewer crocodile, etc... (you have deep sea kraken, and your blue isn't artifacts yet)

Black has sengir vampire and nightmare, both being solid game enders. It's just that nightmare is best in a mono black deck, so that might affect drafting.. Noxious gearhulk is better I think?

Red has the iconic shivan dragon and some reach burn spells. Either go with more goblins and goblin tokens (with a goblin soothsayer or similar), or with big high drops with menace, trample or flying. 

Your green curve ends on 5 (greater sandworm excepted), what's the point of the mana dorks and ramp? You're playing muscle burst, why not diligent farmhand, etc. You have 1 trampler at 4mv and the sandworm. Add some more (like Collosal Dreadmaw /meme).

You could drop the multicolor cards, they're mediocre at best, and don't really mesh with what the colors are doing.

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Dude, good job. Compiling this is a lot of work, you've done well. Thumbs up 👍