Some Twitch Buffs
Twitch is in a fairly good spot balanced wise and has a unique identity of a hyper marksman opening fire from Ambush to Spray and Pray the enemy team to bits and make them suffer with his poison and Expunge. (I know E and R got name changes, which are lame, I like his old spell names a lot more). The buff I will suggest is more a quality of life function; even though it will be a direct buff to allow Twitch to be Twitch more often.
Expunge "Contaminate" (E): Damage changes to 20/35/50/65/80 plus 1/1.5/2/2.5/3% of the target's maximum health (+20%AP)(+25%Bonus AD) as physical damage per Deadly Venom stack. [from per Deadly Venom stack the stacking base damage being 15/20/25/30/35] This is a late game buff with a % max health instead of flat base damage for the stacking damage, the flat base damage and ratios are the same; % damage makes more sense as a hyper marksman.
Spray and Pray "Rat-Ta-Tat-Tat" (R) Cooldown: Increased to 150/125/100 seconds [from 120/110/100] Passive: For each second a champion is poisoned by Deadly Venom Rat-Ta-Tat-Tat's cooldown goes down by 1 second.
EDIT: Added a change to Expunge "Contaminate" (E) having the Deadly Venom stacks be % damage instead of a flat number; the static base damage and Deadly Venom ratios will remain the same