I suggest (and have before) a bonus to characters that are much loved by their player.
At Vet level 100 you get 1 bonus enhancement slot.
This is quite achievable by standard players who stick with one character. It is not game breaking. To me, it seems like a reward in standard with the other types if rewards a character gets. It is, of course, a completely different type of reward than currency, badges, accolade powers, incarnate, etc.
The goal here is two fold. To encourage the long term development of “main characters”, which i believe 1) increases a players connection to the game 2) increases level 50+ play. The second point is debatable whether that is needed, but it is a goal and i believe this would further that goal.