Hm, you already get a bonus if you don´t die in a quest. And you get a bonus if you kill lots of enemies (conquest bonus)
I wouldn´t like giving kill bonus per character, as that would punish healers, tanks and would promote even more zerging when everyone want to go first into a room and nuke everything to get more XP.
Also, due to the TR hamster wheel, chances are that there are groups at every level unlike other games, where after a certain time, everyone will be stuck in the endgame content.
So I don´t really understand what level you are stuck at !?