I came up with an idea for Doomsday, but it might be too complex/encouraging of comebacks or turtling.
Hyper-Adaptation:
Doomsday evolves and adapts as he takes hits (or blocks them, perhaps). He gains points above his special meter at certain thresholds of damage, indicating that he can use his trait button and a directional input to "evolve" a single special move (depending on the input). Moves gain benefits specific to itself, such as increased damage or changed properties such as armor or comboability.
Done well, this could create a lot of flexibility and variety in Doomsday play, which'd certainly offset the idea that most people have that he'd be a very boring character.
Implement poorly though, and it could end up being... well, DARK DOOMSDAY RISES.
But I thought it was a neat idea, at least.