Right now, Venom Cask applies a piddly two stacks of deadly venom.
What if, the longer 
held onto the ability, the more stacks it would apply? Think Corki R, but instead of number of rockets, it's number of stacks his next Cask will apply.
Twitch could bide his time before assassinating a squishy and pop out with a four stack Cask! I say four because it would need a cap.
I'm thinking minimum 2 and max 4 stacks.
AzureInTheDark1 votesI'd have preferred an additional effect to his Venom Cask that roots his enemies for a set amount of time if he hits them while they have all 6 stacks.
Casri1 votesInteresting idea, I actually misunderstood you at first glance, I thought you meant changing his W into a channel/cast time (think Poppy R) and increasing the strength based on how far along in the cast time he is, which could also be cool, especially if it didn't break stealth. Either idea would give his kit a new dimension, so I'm in favour of it.
DorkunedAuras1 votesI think the problem with this idea here is that venom cask is hardly a "use on CD" spell, so basically every cask will be a 4 stack one.
Omiwom1 votesTurn up the aggression and force Twitch to waste a 2 stack cask to slow your advance. Besides I don't think 4 stacks isn't enough to make the spell oppressive. It's just enough to make it do something. You have to be pretty dumb to let Twitch auto you 6 times in lane, or at any point in the game unless you're a tank. He doesn't have any sticking power which is why he needs this burst application of his passive.