Even after many years of speculation & rumors (not just in the theme park fandom, but in the general Nintendo fandom),
this has always been the most logical expansion path for SNW in my opinion. The LM games have stellar theming and world building, and Luigi is Mario's brother. Their images and worlds are intertwined forever.
Neither Pokemon nor LoZ really fit with Mario--they make much more sense in the other parks (and their own lands). And I say this as a huge fan of both LoZ and Pokemon who
really wants to see lands themed to those two IPs somewhere in UOR--sooner rather than later, lol.
Pokemon has the aesthetic but not the same energy. It's also a very modern-feeling IP, not so much a "fantasy"--the different lands that each main Pokemon game take place in all have small towns, wild areas, and a large, modern city. They've always been themed to and inspired by
real countries and cities. The Pokemon universe is entirely separate from the Mushroom Kingdom universe (which I count DK as part of, given DK was terrorizing Mario back when Mario was only known as "Jumpman" and the SMB universe didn't exist; he predates Bowser).
Legend of Zelda only has three titles that have a similar aesthetic to Mario--Wind Waker, Link's Awakening, and Echoes of Wisdom. While Wind Waker is
very popular in the fandom, when Nintendo fans talk about an immersive theme park land based in Hyrule, they aren't talking about WW or LA. They're talking about "mature" Hyrule--a very obvious Hyrule Castle, Ganondorf (in whatever form he takes) as the big bad, and Link/Epona adventuring across the land while trying to help Zelda. Those are the "classic" LoZ themes and images that stick with fans, and that's what they want to see in a theme-park version of Hyrule.
That vision of Hyrule (including Twilight Princess, the game the stained glass windows are inspired by) absolutely does not mesh thematically
at all with the current SNW/Mario theming. And Link has never been in a SMB title (Smash Bros and Mario Kart don't count in these conversations), so he has no natural relevance to the world that Nintendo/Universal can tie him to, other than being another popular Nintendo IP. Also, Hyrule is grand enough and LoZ beloved enough to deserve its own, separate land--as with Pokemon.
For theme park goes/entusiasts, "crossing themes" doesn't seem like a big deal, especially when they just really want more Nintendo lands to play in. But for Nintendo? Those clear rules are very important to each IP--and to Nintendo as a larger brand and company.
In his most recent english-language interview at the grand opening of Epic, Miyamoto told the interviewer that Nintendo views their "worlds" as individual experiences (he says a lot more, but that's the gist). So, per Nintendo, Mario & friends stay in Mario-related worlds; Pokemon stays in Pokemon; LoZ stays in LoZ; Metroid stays in Metroid, etc.
The
only IP that doesn't follow those rules is Pikmin, because Pikmin exist
everywhere. That's why there are Pikmin hidden all over Epic waiting for guests to discover. In the world Nintendo has created for Pikman, it makes sense that you'd find a Pikmin in a corner of Berk, or Celestial Park, or Dark Universe (I think the only place you won't find them is HP, for obvious reasons), because Pikmin pop up
everywhere. It's part of their core worldbuilding.
So, yeah, TL;DR--Luigi's Mansion was always going to be the expansion, because Luigi is part of the larger Mario world. He
makes sense there, and can be given a fully themed mini-area that flows effortlessly from SMB. Luigi will stay with Mario and in the Mushroom Kingdom and that's what fans (and Nintendo) want.
(ETA: this all might be irrelevant in parks outside of UOR, where space might be an issue; I'm talking specifically about the UO resort and Nintendo IPs coming to each of their three parks. There's no reason to cram everything together in one land in Orlando--they have the ample space and parks to play in.)
Other than theming an experience around the recent Peach-starring game, "Princess Peach Showtime", Luigi is the next most obvious fit, and the Mansion games are
perfect for a dark ride/fully-enclosed mini-land area.
Oh, and
Polterpup merch will absolutely sell like gangbusters in this park. He's adorable.
