It's not that likely, but he could use moves from Mario&Luigi and not Luigi's Mansion, but since Mario got the FLUDD, it's rather unlikely. (Also a stage)
I agree, it's more likely Luigi's Mansion, but why should they chance the basics of the stage and let the stage look like something else? Changing the background color, adding fog and go on.
I think it's most likely a stage not mentioned before here.
And Luigi's Mansion is the best choice for a stage for Luigi (better than his crappy Meleestage..)
I could imagine Ashley being a background element of her castle. Casting spells, let the Pirahna plant grow. A Diamond City stage would be the best second stage for the WarioWare series. It's like the Sunshine stage, but you're going to the place where the protagonists of the series live/work/whatever.
(Mona - Monapizza, Ashley - Ashley's castle, Kat&Ana - The castle/jungle, Jimmy T. - disco and go on)
Maggirus