I agree that it's likely for demo purposes. This series is pretty hit or miss with certain things, but the console games usually keep the roster in-tact at least.
I'm more worried that they'll continue dumbing down the Dream Events.
https://nintendoeverything.com/nintendos-player-research-showed-luigi-was-an-attractive-lead-character-to-a-female-audience/
It's official: The ladies love Luigi. :luigi:
I'd make a joke about Bowser being a free man or something like that, but I can't after Mario Tennis Aces gave him that outfit.
Also it's funny, considering that the reason costumes were shot down as far back as the first game is because certain events would require Sonic to remove his shoes and...
If it's anything like last game, the rivals will probably be guests.
I'm hoping I'm wrong and we can use them in any event though (or at least make Bowser and Eggman Nega skins for their respective characters. It can't be that hard.[/spoiler]
https://nintendoeverything.com/new-luigis-mansion-3-details-next-level-games-confirmed-as-developer/
Some more info.
Biggest take away is that the mission structure from Dark Moon has been dropped and we'll be seeing more ghosts in the vein of the first game rather than Dark Moon's take of...