I think hero synergies between 5* and 4* heroes would be great.
Ie The 4* hero gets a permanent bonus or boost if fighting with the 5*
All enemies receive 190% damage and lose 30% mana. Mana regeneration of all enemies are reduced by 10% for 3 turns.
Synergy: If fighting with Little John, Little John’s defence and health are permanently boosted by 20% for the duration of the battle. Little John’s mana speed also changes to average for the battle