Take your identikit teammates, for example - something that we were a little displeased with first time around. Sure, you can designate attack/defend behaviours to your team, but this is scant consolation for not being able to pick - schoolyard-style - from a roster of Nintendo All Stars. Having a team of five distinct individuals would have made a massive difference to the longevity of the first game - particularly where changing strategies was concerned.
Developer Next Level should also look at Golf or Tennis to see how Mario-themed stages are really done - a repeat of the drab Underground or Crater field would be a disaster. Likewise, some more variation in the way the pitches actually played would be nice too. The differences in the GC Strikers were pathetic at best. Slight differences in size and ball bounce were all we had - surely it doesn't take much effort to sort this out?
Oh, and finally, here's another tip for Next Level. Any game that proves more fun in multiplayer mode with a cheat activated (Weak-Goalies cheat for added fun!) is a definite sign that there's something a bit wrong with your game...