I believe the problem in twitch is that he gains natural speed attack increases just from coming out of stealth AND using his ult.
This allows every twitch to greedily stack Crit damage runes and use Crit chance items in game, with absolutely no regard to attack speed items.
This is what, I believe, results in his insta-killing of multiple people late game. If you die to a twitch when he's not using his ult, it's fair game imo(at least until he hits the 3IE mark due to his 30 kills). Twitch is a lesser ranged + weaker poison than Teemo without his ult, so breaking his ult breaks the character.
But forcing Twitch to stack other items as well as crits will keep his abilities useful while reducing his damage.
Therefore, to fix twitch you must rid completely of his attack speed gained by his ult, 120% is just too ridiculous when stacked with his 60% from leaving stealth. I don't know if these stack honestly, but if it does 180% attack speed is just too much (that can be obtained frequently, mind you).
A little drastic, i know, but Twitch players WILL manage without that 120% attack speed- for they still obtain 60% for leavnig stealth.
Less attack speed = need for attack speed items = less full critical builds.
My (fail? if so, please explain...) solution for twitch...