Not entirely sure how the stamina cap is calculated. I assume you gain a higher cap as your team level increases.
Just an idea - can the team cap be increased by an amount:
- each time a hero of yours reaches gold, platinum (silver and bronze ignored) ; this shows that the player in question is dedicated to the game and promoting heros takes a heap of time and so additional stamina in recognition of this commitment makes sense)
- each time a hero of yours reaches 6, 7, 8, 9, 10 stars (5 and below considered too low for reward)