Prestige Rewards
I understand the want & fun in getting your characters to prestige 100 but besides the intrinsic rewards you get from just playing the game, what reward is there or actual purpose for getting to prestige 100? It's almost incentivized to just prestige characters to 9 and move on to the next as to unlock everything.
Only suggestion I've come up with are releasing alt legacy cosmetics; i.e. look like legacy but instead of gold/yellow make them black and smokey or like the entity) at the later prestige's. For example P80 you'd get the torso, P90 the legs and P100 the head/weapon.
Also you could release special badges for each character unlockable at P25 & A banner at P50.
Just some food for thought

Comments
I mentioned this in my post as well. I do think there should be some rewards for reaching high prestige levels.
A twist on the legacy cosmetics was something I suggested too, and something that could be great as a reward for being a dedicated player.