I've played the cube only three times now and I've already learned so much just from drafting. There's a couple of things that I think need tweaking:

• Biggest change is that I've removed what I consider the worst of each of the mono-colored planeswalkers (Teyo, Lili, Jace, kid Chandra, and Vivien)
• I replaced those walkers with low-cost creatures that I think will increase the speed and of the cube and give faster decks a chance.
• Bioessence Hydra is just too much. If you have even one planeswalker you'll start out with an overwhelming advantage. Genuinely warped the entire game around it at 5 mana, defended the planeswalkers and buffed itself, just a ridiculous win-more card
• Blue-Black is getting reworked, not because I played that much against mill but because I recognized that mill wasn't really enjoyable to play. You either land Jace Memory Adept and mill them 20, probably winning on the spot or you don't have one of the like 10 cards that support mill and you lose.
• Blue-Black is now about tempo & ninjas, essentially trying to attack with unblockable creatures. This gives them a sort of controlling role that wants to cut through the large boardstates.