Actually Tristana got hit really hard on this last patch:
She gained 20%AS on her Q at rank5, but lost 27%AS at lv18... so -7%AS overall... so she's less of a lategame hypercarry, no matter what you do. AND you suffer to get there. You are crap early game, crap mid game and now, thanks to Rito, you are crap also late-game because there's no whay you'll be melting tanks with new Randuins in less than 5 seconds (Q duration). And good luck even reaching lv18...
Also, Randuins got buffed, which means tanky offtanks will be all over her and she won't be able to retaliate. She will have to concede.
The only thing she has going for her after all the recent nerfs is her range (nerf on E base damage, nerf on R cooldown, nerf on magic Burst at lv6 (which she has no more), nerf on AS%/lv, nerf on Q duration, Q dependance... so Vayne and Jinx are going to eat her alive...). And ultimately her range isn't way better than Kog (bio-arcane barrage), Twitch with Ult, or Jinx with rockets.
Instead of nerfing her, they should maybe balance the items that swifted the meta? (BF Sword end items, BotRK)
Tristana only got "good" because of the nerfs to Defensive Items like Randuins and the buff to BotrK and BF end items. Balance them back, and you'll see current Trist is crap.
I'll remind you the spirit of Tristana was to be average in early game (with her high based magic damage) until lv6/7, crappy in mid-game with her lack of power/range/AS... and MONSTER-LIKE in late game with her Range and AS.
Now she's just crap. Crap early-game, even more crap mid-game (snowball effect from bad early) and not-as-good-as-before late-game, good luck even getting there...