Since energy can be a limiting factor on doing things in the game, I would like to see an endless waves mode or something to that effect in the game.
Basically, you’d have your team and start off with easy enemies. As waves were cleared, perhaps a boss could show up every 10 or 20 levels. Items would be available, but limited as if it were the world map. Also, there would be no energy needed to start this mode, as starting from the bottom and working up again would be a bit time consuming in itself. The amount of bosses defeated or waves defeated could give a chance to gain some ascension rewards to perhaps remedy the prevalence (or lack thereof) of some of the items needed to raise epic or legendary heroes.
If cost would be a limiting factor, perhaps play an ad once a boss is defeated or have VIP be able to bypass the ad?
At least this would give players something to do if they wanted to zone out on their phones for a bit, since this would be a great road trip passenger mode. I don’t think this would detract from the other play modes since those rewards are a bit more guaranteed.