We can learn from another online game when trying to balance Tristana.
Just look at TF2's Soldier. The same piece of crap as Tristana is right now. He jumps around, does high damage, and seems unkillable at times.
How is Soldier balanced out? His rocket jump self damages him.
I mean it just makes sense right? Shooting yourself in order to jump higher SHOULD damage you, and it balances his power out nicely.
A Soldier has to consider that by taking a rocket jump he is giving himself a health disadvantage, and needs to play around that. A good Soldier that constantly rocket jumps has to find healthpacks and ammopacks around the map to stay sustained.
Now how can this help us with Tristana? I propose that we give Tristana a health cost on her W: Rocket Jump. This means that by going in OR escaping, you are potentially harming yourself considerably if you have poor decision making.
Think about a Tristana confronting a Veigar. Due to the nature of Veigar's R, he will do considerably more damage the lower she gets. Tristana has to choose between staying safe and walking up to Veigar, or jumping in and possibly being insta-bursted.
I'm really tired of Tristanas just blindly jumping around because they can't get punished for it unless they literally jump straight into the entire enemy team. Or Tristanas spamming the reset to get their free penta. The last person to die should have a chance to turn the fight.
I think a health cost scaling with level from 50-140 damage would be fair.