Tristana's "Rocket Jump" ability should make her invulnerable until she lands
This is just my opinion, but if an enemy champs ability is targeted and fired at Tristana, and she jumps, there's just no conceivable reason why I should be able to watch a fireball follow her into the air and then hit her. Additionally, if an enemy champ stomps the ground for an AOE stun (for example) and Trist is already in the air, I can't even imagine why she would be stunned by that.
Just throwing this in, but I don't think any other champ has this problem, and champs like fizz use similar abilities and ARE invulnerable for a much longer period of time... just sayin.