Personally, I enjoy fighting games a lot, but I have to agree that Playtonic should rather focus on platformers. There are quite some good fighting games out there without them contributing, while the platformer section could really use more love.
I so want them to try a platform fighting game. Think of Super Smash Bros and Digimon Rumble Arena. It's a subgenre that needs more love. Not to mention Yooka and Laylee would be perfect as unlockable characters.