So regeneration would:
1. at the end of each battle won, resurrect any hero with the regen buff to full HPs and avoid any wounds from this combat only, provided they won the battle outright.
2. At the start of each season (before any actions), set any hero with regeneration active to full HP.
3. not give any HP back in battle
4. have a higher mana upkeep then 1 per turn.
5. the hero still wouldn't get any XP from the battle to balance being automatically set to full HP, but as long as their side wins outright, they won't suffer a wound.
So you'd still have to win outright, but if you did and a hero with regen was lost, they would be resurrected with full HPs and no extra wounds. It also means that you have to bring enough firepower to any battle for the regeneration to work.
Troll Charms would probably still heal 1HP per action.
What do people think?