This game need some kind of challenge mode just like those in KingdomRush.
I think the biggest problem with this game now is that the number of missions is too small, and people will just get bored soon after playing them again and again. Adding more missions and heros will help, but that’s a lot of work. Adding skirmish mode will be nice, but that doesn’t sound easy.
I think the best way to solve the problem is to add challenge mode. It can be unlocked after each mission is completed, the content can be something like “We have conquered this region some time earlier, but the enemy is fighting back. Survive as long as you can!”, “The enemy have took back the region. Face stronger enemy and clear the area again.” It won’t take much effort, but perhaps this will let people play longer before get bored.