For me its a theme park as its opens up a lot of possibilities for level designs. It also would probably give the series the Open-world treatment so many games have been getting lately.
I was thinking more around the lines of Odyssey or Kirby ATFL where their open but still somewhat follow a linear path.I don't want Luigi's Mansion to be open world, personally. Just because Sonic and Dynasty Warriors did it doesn't mean it's good for that game.
I don't know about returning to Evershade valley again as its seems kind of small to make a city out of. Plus we only have 3 games in the series, it be weird to drop King Boo now considering he's one of the only few consistent characters in the series.I think they could expand on the idea of Luigi's mansion dark moon and keep the multiple mansions concept but make it open world. Think of it like this:
Evershade valley became a huge tourist attraction after the events of dark moon and people turned it into a city. Evershade City. However the ghosts that lived there hated this and attacked. The dark moon which was the power source of the city lost it's power and it is up to Luigi to solve the ghost problem and the energy problem. The problem is solved by the ghosts being an energy source because it is revealed that in dark moon they can be tracked by the sheer amount of energy they emit. It is revealed that one can simply harness the ghosts energy by building a device that doesn't even hurt the ghosts. ALSO!!! E Gadd is the villain. Well sort of. He could of made an invention that didn't work. It is what disturbed the ghosts and used up the power source. And then comes the problem of king boo. Even though he is supposed to be the main villain of the series I don't think he has any more use. He served his purpose and worked but I think it's time for Nintendo to drop him for a few years like they did with boom-boom and the koopalings in the 80s and 90s. Anyways this was long and if your read all of it congrats you aren't a lazy idiot.
While yes, he is one of the only recurring characters, adding a variety of villains would be nice. Where would the Mario series be if Bowser was the ONLY villain. Also evershade valley is supposed large and it's just we only get to see the mansions. Even with the fact that we only see the mansions the fact that their climates differ drastically we know that it has to be the size of approximately the first two worlds from Mario Odyssey at a minimum.I don't know about returning to Evershade valley again as its seems kind of small to make a city out of. Plus we only have 3 games in the series, it be weird to drop King Boo now considering he's one of the only few consistent characters in the series.