Content Idea
Hi,
I had this idea called "solo quests" how it works is you must own a +3* of said champion and go through quests taking only that champion. One of the reasons for doing this is to get an idea of how said champion came to be. Show some comics/story, and learn how to play that champion, find out their abilities etc. An example would be getting a wolverine (from a crystal), learn about the weapon x project, and use wolverine to escape the laboratory.
As for small changes have the energy you get from levelling up add to your current amount instead of worrying about wasting energy refills, and to attract more activity have double energy (1 refill fills twice the stamina) refill maybe 1-2 times a month with an odd double Xp weekend.