Simple Idea to Fix Twitch

Instead of Q granting bonus attack speed upon de-stealth, Twitch reappears in a burst of noxious smoke that deals damage in a ~475 unit radius and applies 1 stack of venom. Not too much damage, like maybe 50/80/110/140/170 + 50% bonus AD. This effectively means Twitch has to actually sneak up close to enemies to do maximum damage (like other assassins). It gives opponents a chance to counterplay, since the smoke blast radius is smaller than the stealth radius. Though Twitch can still sneak into teamfights and ult from 850 units, his effectiveness playing that way would be diminished without the free attack speed steroid. He can choose to use the Q smoke burst damage in teamfights, but he'll have to be very careful in positioning, timing, target selection, and maximizing the cooldown reset mechanic on his Q; all of these encourage skill expression and flashy plays.

He'd be somewhat like Ahri (though still very different; there isn't really a current champion that would make a suitable comparison), where he can choose to stay in the backline, or go all-in for high-risk assassin-esque plays.