Grid/SportsMan's stage seems easy. I already figured everything out on the first run; same goes for SheepMan's stage.
Actually, I wouldn't mind it being a little easier, since that'll be fun for speedruns, but it's hard to see people having difficulty with this since MM1-3 threw either comparable or greater challenges at us already.
Oh yeah, and what's this goalie midboss' weakness, anyway? Thunder Wool will go in the air, right? Maybe that.
EDIT: Whoop, I nearly forgot, I LOVE how the Football actually has a different bounce pattern than the baseball. LOVE IT.
And it looks like ProtoMan's going to play a huge role, on the title screen and everything!