A nerf Trist needs is to actually have her unable to use her jump in the time frame she's disabled

Well they did buff her to compensate her for the crit item changes when riot was experimenting with crit.

Then they reverted / buffed crit items without nerfing her.

Alternatively, reduce her safety, while encouraging her all-in playstyle:

• Base attack range, Explosive Charge Range, and Buster Shot range reduced to 500 (was 525)
• Base AD increased to 65 (was 61)
• Base AS increased to 0.665 (was 0.656)
• Base health increased to 570 (was 559)
• Range bonus from passive increased to 0-160 (was 0-136)
• Rocket Jump cast range reduced to 750 (was 900)
• Passive now applies to Rocket Jump cast range
• Rocket Jump damage increased to 120/165/210/255/300 (+50% AP) (was 95/145//195/245/295 (+50% AP))

These changes would lower her early game safety, but increase her overall damage. With very short attack range, she would have difficulty getting good trades in early on, and the lower range on her W makes escapes harder. However, the increased base stats and W damage make her all-ins even more deadly.

Tristana is working right now because she is far too safe for an ADC. Same goes for Kai'Sa and Ezreal. Their mobility allows them to get out of danger that an ADC has no right to get out of without a Flash. Instead of just hard-nerfing them, they should get compensated with buffs to the things that make them fun. For Trist, that's all-ins and resets.

For Ezreal, they should probably look to make his Q CDR not affect his E, but have his E reduce its own cooldown when he lands it on a champion. Maybe make it a mechanic unique to him where each of his abilities reduces their own CD when they hit stuff, instead of his Q reducing everything. He should be rewarded for staying in combat and making ballsy plays, not just poking and escaping.

For Kai'Sa, I think they should make her E provide little to no movement speed early on, but increase the shield on her ultimate, the cast range of her ultimate, and let her cast it during the E channel. Her E should start as an awkward channel that slowly evolves into a dash, not a dash that evolves into a better dash. Improving her ultimate makes those all-in plays more consistent, and casting it during the E would not only help make up for the reduction in mobility in the early game, but it would also open up awesome moments where her evolved E lets her disappear entirely and suddenly reappear behind you.