With all love for his great qoutes, asking people for sandwiches and saying hello, Twitch is a great champion. He is for me the only champion in the entire game that is fun to play even while I am tilting because every other champion can beat him. Even my friends hate it when i pick him because we are sure to lose the lane.
Pretty sure we can all agree his early game is far worse then Vayne's or any other ADC. His lategame might be great, getting there never happens due to matches being decided before 10 minutes. Not to mention that IF there is an enemy Zed, you might as well be AFK, because at least you are not feeding that guy.
Twitch loses from anybody that is tanky, has a ton of mobility (everyone these days) or huge range (also includes all latest champions). He also dies quickly to burst but considering hes an ADC that is supposed to happen. CC hurts any other Carry as well so pretty obivous that's also a weakness.
So I decided to make some changes to make Twitch a bit more reliable. First off is his passive, which deals true damage per second per stack. The damage is so slow the passive basicly reads: "Your basic attacks apply one stack of venom". Instead of that crappy true damage per second which does not fit any carry at all, why not change it to an On-hit true damage. Let's say that every stack on an enemy allows Twitch to deal 0.25% of his enemies max HP in true damage. That's only 1.5% true damage per AA but it shoud at least help a bit in killing tanks. Or anything really.
Or you give him like a 2 or 3% damage buff per stack, which would allow Twitch to deal more physical damage. Either way, a weak poison that can barely outdamage a champions base regen is downright bad.
His stealth is fine, altough it would be nice if stealthing after a kill would always go in 1.5 seconds. It's nice you got the reset, but with all DoT's going on it takes a tad too long. Maybe even make it instant, like Vayne's but massively reduce the duration.
The flask is just... Maybe add the grounded debuff Cassio has for 1 second or something short so you can at least get one AA off before that assassin jumps on you. And for the love of god make it so that it does not interfere with your AA. You lose more than 1 AA while casting this, which durnig your ult is quite a lot.
His Expunge isnt bad at all, but maybe giving it bonus damage against minions and monsters and allowing it to regain a bit of mana after killing with it would work. Like Kalista's rend. The bonus damage against monsters would allow him to jungle early a bit better, which I don't think would cause any issues.
Twitch's Ult is basicly the best thing in his entire kit, as it should be. But it kind of sucks that a fast champion like Yi, or a mobile assassin can dodge your AA's by just moving to the side a bit. Although I can expect that those homing bolts might be a bit too much.