If Twitch is going to be paying 90 mana It should be doing more damage.
It's already been said before about Kalista's E and I do agree with you guys about it.
Twitch's E should be buffed. Other wise lower the damn mana cost. 90 mana is almost like blowing your ult. When you use both E and R you are using up more than half your total mana.
If Contaminate does get a buff I think one of these ideas below would be very reasonable and it will help Twitch's problem from being shut down too easily. pretty basic if you ask me and it's reasonable.
Higher scaling per level Higher base damage including base damage per stack. or do physical damage over 4 seconds.