NOTE: The idea in this post was considered not good. See post #3 for the real deal.
I'd like to make Twitch more of a glass cannon, more counterable and more complex to use while retaining his deadliness. Note, I do not play Twitch, I don't have him, but I did play against him and on his side a couple of times.
Let's make it so his Ult drains his HP. A moderate amount per shot (scales with his Ult). Say, 10/15/25 HP per shot. The numbers are just examples, of course.
This wouldn't be handled as damage, but as negative HP regen, so Kayle couldn't shield him from it. If this is possible at all.
This would not only make him more counterable, it'd make him harder to use. You couldn't go all out attack speed and damage and steamroll players. You'd have to balance your HP, HP regen or lifesteal, and attack speed items, so you don't die when you use your Ult.
His Ult wouldn't be toggleable, but still cancelable, but not by himself (or it'd be, if that'd make him die too often).
Either way, he'd do less damage to your team:
He'd have to get more healing items, so he'd get less damage or attack speed items.
He'd go pure DPS, but then you could kill him faster, thus reducing his damage.
He'd shut down his own Ult as to not take more damage, again, dealing less to you.
And so on...
What do you think about this? I think it's a pretty solid idea from what we could work from. It also seems better than simply nerfing his range or his abilities.