While I am a SPM supporter and fan, I don't really want a sequel of it.
I don't know it just feels like a one-time game. Would be weird if they decided to get back to its concept. It should just stay its own thing. And I know Galaxy got a sequel, but... um... it's a main series game... and honestly did it need a sequel?
Now I do agree that the 3D mechanic should've been worked with. It has a limited time (although you CAN make this time unlimited, but who's gonna beat the Flopside pit twice for just that at the end of the game.) and the environment kinda also feel limited to one long rectangle that lacks width. But they also managed to shove many secrets that I actually only discovered over the course of many playthroughs. And the villains, while their design is questionable (but I'll hand it out to Mimi and Dimentio for really standing out) , are pretty likable to me. And just like it was said by FTG in another thread, they actually went for the geometric designs to make us feel somewhere else. If they wanted to instill that feeling to us then they nailed just that.
And some things look really cool in that style too.