In many other RPGs, a hero can gain experience through battles. What if EnP heroes were able to gain experience (albeit a limited amount) by being selected for any non-titan battle?
For example, a hero could get:
5 XP for being on a “battle roster”
5 XP for surviving the battle
10 XP for each “kill shot”
10 XP for each time they fire their special
And perhaps cap the XP earned per level at 70, which is just less than half of what non-color feeding a one-star hero to another hero earns. Feeding should still be the main way of getting experience, but this would be a slow road to helping players improve their teams.