Thinking of Forgotten Mario Villains:

 Super Mario and its spin-offs have multiple villains, meanwhile main Mario games be like:

Oh noes! Bowser has taken Peach! Call Mario!

The first exception being Super Mario Bros. 2, aka Super Mario USA, the reskinned Doki Doli Panic. Super Mario Land has Daisy as the victim, kidnapped by the Alien Invader Tatanga. Super Mario Land 2: The Six Golden Coins, has Mario losing his castle to Wario. Tatanga returns as a level boss.

Wario Land: Super Mario Land 3: Has Wario hunting down a missing Peach Statue in order to get a reward. There he fights Captain Syrup and her Pirates.

So, Nintendo has a treasure trove of Non-Bowser villains and I wish they'd use them. 

They literally have a Child Friendly Freddy Krueger and his latest appearance was a Cameo in Link's Awakening.

Tatanga screams MARIO KART!! Yet we get a stupid cow!

Seriously, Nintendo, what the fuck!?

But here's the thing: Mamu(Wart) could easily come back. In fact a game where MAMU and KOOPA team up to defeat Mario and friends is the "perfect story".

Something, something, Bowser finds a way to communicate with the Evil Mamu of Subcon. This mcguffin that allows the communication between worlds, allows Mamu to create obstacles from the dreamworld into the real world. So, Mario and pals need to juggle between the waking and sleeping worlds to beat Bowser, Wart, and their teams. 

Sounds a bit Link Between Worlds...

Actually the inspiration was a different game.

At least for single player mode. On this 2D side scroller, You get up to a point where you can sleep in the waking world and enter subcon to clear obstacles in your way on the waking world and vice-versa.

In Multi-player mode, the teams can divide the tasks between raiding subcon and the waking world. But in order for everyone to reach the goal, both teams would have to swap worlds in order to succeed and both teams reach the waking world goal.

The Waking world would operate like a hybrid between SMW and NSMB. Subcon would be a NSMB/SMBUSA hybrid.

This idea is kinda canonical with the subspace mechanic from SMBUSA.