Twitch should
be able to use Rylai as a defensive item, his passive deals magic damage and has an AP ratio. It does damage overtime like morde's ult, it basically is a replica outside of being able to capture a soul. Also, why can't he stack infinitely? He is immobile outside of stealth and his stealth has a 16 second CDR. Kalista can stack infinitely, and is played widely. Twitch would also be played widely if he was given the same treatment as Kalista.
"But Subject 5, he does true damage with his poison!" Yeah, that is kind of the point. He is not an early game power, nor a very strong mid-game power unless if he is somewhat ahead. Even then, Vayne does true damage based on how much HP that a person has. Then she can build bork. Twitch is not a duelist and requires a team with good peel, he should have something going for him just as much as Kalista.