Encyclopedia
Just to make peoples life easier and as a fun distraction I think an encyclopedia where you can view every hero thats out. Or to take it to another level a place where you can level them upp to see how much they scale so you can play around with builds without actually having to spend ISO or covers. Might save a lot of frustration to newer players if they dont know how to best level their dudes and its just fun to dick around with. Also it would for sure motivate people to try to get covers once they see how badass a Wolverine Patch is at max level or whatever.
Stat-tracking
For competitive or just as a fun distraction to see how you play.
For competetive purposes it can be things like highest damage done in one turn of which you have a global leaderboard for, or number of tournament wins, number of top 10 finishes etc. Maybe a top 100 or something. For your benefit it will keep people playing for sure and for players it makes the game more fun.
As a distraction for personal stats it motivates you to outdo yourself, even for people not trying for a global leaderboard thing. A great example of that is battlefield or the lesser known game on PS3 Warhawk. You get those silly stat medals and you try new things and its just fun. Examples of stats that could be tracked are, most used superhero, best team, most used skill, tournament wins (so you can compare to global leaderboards), total health packs used. Or whatever I’m sure you would have a lot of ideas.