Suggestion: Arena in each kingdom which allows players to duel other players in their kingdom with no losses.
Mechanics: use the tactics school injury and energy model. No unit deaths, cost energy, free spells. Can be a building next to the capital, or another tab/button
1. Increases engagement of players. Allows players to experiment with full power armies, formations, hero tactics, etc.
2. Because this is within a kingdom (different colors cannot duel each other in the arena), real PVP will still carry the same risks. Real PVP will be more fun because players will go in with real strategies
Bonus (does not need to be implemented immediately)
1. Include arena rankings
2. Fight records (e.g. "Overall: 60 wins, 49 losses". Or "vs lich: 2 wins, 35 losses, vs human: 38 wins, 10 losses, vs dwarf: 20 wins, 4 losses")
3. Fight rewards
4. Allow players to bet their own resources (big potential for exploit here, but would be fun!)