I think we can all agree Tristana is the premier ADC (or at least right up there) at this moment in time, so how best to nerf her to keep her viable but cut back on strengths she shouldn't have?
So what strengths should she have?
I would say super safe long range late game oriented ADC. Who also wrecks towers.
And what weaknesses should she have?
Early game. Range plus a jump on a late game ADC is too much to have a laning phase that's not even weak.
About her overall theme?
How is she too strong?
Laning is waaay too safe with a jump, range, waveclear and even strong trades.
Where is she actually balanced?
Mid-late game fights and tower destruction - it's what her design screams for her to be good at, and she is. Without being Twitch-tier (Q, R, penta) at teamfighting.
So how to pull those strengths and weaknesses together:
Maybe not all in one go, but it's just some thoughts.
1. Change the numbers on her passive. She has average range at level 1, long range by level 6 and ludicrous range by level 16, even if you don't buy RFC. So I would say have her base range culled right down maybe even below 500 at level 1, but still scale to the same level in the late game. It would give her a weaker laning phase, but not impact her mid-late game as much.
2. Change her E passive to scale off bonus AD more and cut back on its base damage. Currently, the passive explosion damage means you have very strong wave pressure just from last hitting, a pressure not many ADC's can match, so you can just shove and force the other ADC to ignore you in favour of not being perma-shoved under tower, which is essentially a psuedo lane bully, on a late game hyper-carry. Cut back on the base passive explosion damage and you reduce her wave push early, but once she starts getting items it will still be there.
3. Change her W range to scale with her passive and/or points in the skill. Currently it's too much to make her gank-able in laning phase and it plays into her being too safe early. Removes a nice chunk of safety and may force skill points from her 2 damage abilities.
4. Make her ult worth putting skill points into. I don't care how, but currently 1 point is enough for the entire game, so those skill points at 11 and 16 can get more damage in her main abilities, which more or less translates as an unnecessary advantage since power that could be in her ulti, is in her Q & E.
Just a few thoughts on where I think Tristana needs to be looked at - basically trim laning phase strength whilst retaining mid-late game power & safety. And make her entire kit worth pumping skill points into so you can't just dump everything into the mad damage on Q & E.