I just think you aren't considering how powerful venom will be with no drawbacks. It will be arguably the best trait in the game. Adding a whole lot more on top of that is ridiculous. A charge cancel, changes to venom, and maybe some very minor tweaks is completely reasonable.
The more I think about it, the more I realize how stupid it is that Venom makes Bane take more damage. It basically gives his trait to his opponent during cooldown. Imagine if Aquaman couldn't combo his opponent while his trait was on cooldown, or if Flash moved super slowly, or if Deathstroke's opponent's attacks became unblockable. I was hit with Ares's super while on level 3 cooldown and it did 55% damage to me, which is actually more than Bane's super does with a level 3 boost. Does that make any sense?
I can understand how difficult it must be to balance, because no one wants to see a character who can just dish out 60% combos with no drawback (well, maybe besides Superman for some reason...), but maybe they could give the various levels of the trait more effects than pure damage bonuses. Armor-breaking, additional juggle potential, damage reduction, access to new normals and/or specials; all of these would be acceptable effects of additional levels of venom.
One last thought: Because of the nature of the Venom mechanic (the fact that you can just mash out up to level 3 at any time), if there was no downside then there would be no point in not going level 3, except as a way to extend the active time (i.e. go level 1, wait, go level 2, wait, go level 3). Because of that, if we want no detrimental effects, it would have to work like Grundy's, Catwoman's or Joker's traits, where you have to build up additional levels of Venom instead of just mashing it out.
