Oof. Okay, let's see how long or short this ends up being. . .
Brexit
Chance: 0%
Xenoblade 2 already has three songs, two Mii costumes and eight Spirits in the game, which cover most of its main cast. Rex' Mii costume in particular was included as an apology for him not making it, with Sakurai even saying that "hopes that it makes you feel like you're playing as him". As Byleth, and, by extension, Corrin have shown, first-party DLC is more likely to be of the promotional variety, and as for that? Xenoblade 2 released over two years ago, and considering that (non-X) Xenoblade games follow the Final Fantasy/Fire Emblem approach to characters and settings, Rex' chances of being the protagonist of another game are pretty much non-existant. There is no shilling benefit to including him. Unless he gets picked up for Smash 6, he's gonna fade into obscurity by the time that Xenoblade 3, or even X2 roll around, and as far as I'm concerned, it's for the better. Which brings me to:
Want: 0%
If I could go below 0% for this want score, then believe me, I would do so without a moment's hesitation. Let me make this clear: I absolutely
hate Rex. The original Xenoblade is one of my favourites games of all time, and Xenoblade 2 as a sequel takes pretty much everything that I loved about the game, and at best sort of does it alright, and worst, outright removes it. And Rex - as well as Pyra - himself is a huge part of why I don't like the game. His design is terrible (go ahead and google the term "bad character design", he's literally among the first results that pop up), and him inevitably coming with Pyra would bring a design that's even worse. His writing is, boring atrocious and generic, from a series that already managed to have a pretty good protagonist with Shulk. His voice is annoying. And to actually mention a point that would directly affect Smash, I cannot conceivably see him having an interesting moveset.
Everyone always talks about how "oh, he could come with Pyra, like a dual fighter, and he could switch Blades!!", and honestly, have we even played the same game? Combat in 2 has the driver doing most of the work, with the Blade only dropping in to do something for some special attacks. More likely than not, any Blade that Rex came with would just stand in the background and hold out their hand at him, only actually doing something for maybe a special move or Final Smash. If you want a proper dual fighter with a similar mechanic except better executed, there's always the Astral Chain protagonists. Anyway - even for Blade switching, I hardly see any appeal there. Who'd you even give Rex to get a full team of three? Sure, Pyra's a given but that's only one character. Who are the other two? Mythra? She wouldn't really work, because the weapon and arts she grants Rex are virtually identical to those of Pyra, only with a light element instead of fire. Both of the other Blades that Rex gets as a part of the main story could work, but both are spoilers. The one moreso than the other, but I really can't see said other getting spoiled by Smash, even with what they did with Fiora. The only other option then is to the Byleth route, and give Rex the Blades of the other party members, but even that doesn't work that well. Pandoria's sword form is extremely similar to Pyra's sword, so there's already plenty of potential overlap. Brighid and Dromarch meanwhile also have potential overlap, since both are dual wielded weapons. I know that Dromarch's rings are visually rather distinct from Brighid's whip swords, but even in 2 itself, they already had no issue in cloning both dual dagger and a dual katana animations from the ring ones.
In summation: Don't want this kid. If he somehow still makes it in, he'll be instantly my most hated character in the roster.
The "X" makes it sound cool.
Chance: 35%
I'll freely admit, I'm most likely quite overrating Elma's chances here. Sure, with my earlier statement about first-party DLC being most likely promotional, she's at first glance in a worse spot than Rex, since X is even older. But Elma simply has one thing that Rex doesn't: Potential future relevance. Sure, there's always the potential of an X port coming, which could already help her chances. But even beyond that, a sequel for X is pretty likely. For as much as people want to call it the "black sheep" of the series (???), it still sold around a million copies on the Wii U, and the series' director has stated that he doesn't like working on the same thing too soon after finishing something, meaning that right now, X2 is looking more likely to be the next game than Xenoblade 3. And considering that the original X ended on a cliffhanger, Elma coming back for a sequel is looking pretty much guaranteed.
With Ultimate itself, it's also important to keep in mind that the game still has no music from X, and that not even all of the game's (non-optional) party members have Spirits. The content to be added for a Challenger Pack is definitely there.
Want: 100%
Elma is currently my most wanted character, to the point that both the base roster for Ultimate and first Fighter Pass have left me rather bitter over her lack of inclusion and content for X. With her dual-wielding katana and gun moveset, she'd also provide a suitably interesting moveset, and something we don't really have in the game yet.
Predicting Skull Kid to get around 9.6%.
Predicting Midna to get around 7.8%.
Nominating Any Octopath Traveler rep x5.
. . .I call this post: "Xenoblade 2 and Rex in particular are garbage, and here is why".
In a strange turn of events there's absolutely nothing to mention about the nominations list.
But isn't that in itself something worth mentioning?