Besides competition events…
What I would like to see is more use for these 3* and 4* teams.
-A separate raid system or/and war system for each class allowing the max * level. (This would force a little more strategy and allowing for a learning curve before moving onto the bigger and better heroes. Another bonus I think this would give is the newer players would have something to do and a reason to start leveling up the 3* before going straight into the 4* heroes or 5* heroes.)
-Questing could be involved with this. Make Quests available to certain rosters levels with different level of rewards.
This would allow for new heroes to be created in the 3* and 4* tiers as well. And spice up not seeing the same old bland heroes day after day and just feeding them away.