Okay hear me out on this 'cause I got this from a dream, but a Mario + Rabbids DLC that's about helping Bwaluigi and Lava Queen find Bwario who's been fiddling with Darkmess and has now gone missing, and the final boss involves breaking him out of it
We need the Bwah Trio back, this is a great explanation to their disappearence after Kingdom Battle!
Bwario messing with things that he shouldn't sounds something he'd do. Maybe not even out of malice or sense of revenge towards the Heroes, but just out of pure curiosity for a powerful thing like Darkmess.
I'm not sure where this would take place: perhaps in the Mushroom Kingdom, since we don't know what happened there during Sparks of Hope. Or maybe Bwario is being brought (or controlled to go) to Cursa by a Darkmess tentacle and we get to play as Bwaluigi and Lava Queen, who need to chase it across all of the maps we've seen in both games. So while they're fighing to save their friend, they would (unknowingly) clean up the last bits of Darkmes the Heroes may have left behind.
