Story mode with a twist: your hp doesn't go back to 100% at the start of each new map, it stays at what it was after killing boss of the previous map. And if you die, you lose and have start over from the beginning. The reward for completing the entire story mode would be a huge exp reward that would be comparable to if you had done a bunch of 258 in the same amount of time (make it a bit less tho). Make exp checkpoints as well, so you get a set amount of exp if you die after passing certain levels, but make beating the entire story substantially more rewarding.
It would be so fun to play all the old maps, and this would be a way to encourage ppl to do it...
Also do some sort of similar exp rewarding system for normal story mode :3