Excellent game. There are a couple of improvements/questions that can be made (in my opinion):
1. Adding trophies
2. The guy you unlock at the end says he has all abilities but doesn't (inserting him onto my team results in a quicker demise in Endless).
3. It's fairly quick to go through all of the combinations and power them all up - perhaps allow the ability to keep feeding them for endless mode? I suppose trying all the different team combinations would take time up but I think I'd still be left wanting more.
The team I came up with was Dark Dragon, Scarface, Blade Spirit, and Entropy getting me the A-level of the final unlock and level 8 on Endless.