If FP5 is a Nintendo character, I think a lot of people are sleeping on Elma's chances, considering Xenoblade Chronicles X is notably missing from the basegame with maybe 3 Spirits for rep (including Elma herself) but no stage, no music, no really anything.
The lead music producer for XCX, Sawano, has a secret project he worked on last year that nobody knows what it's for yet, and Elma's VA is under NDA for a secret role that nobody knows what it's for yet either. There's reference in the Elma Xenoblade 2 DLC content about everybody "finally being together" (or something of that affect) while Shulk is also there that seems to indicate something of that nature. There's also the mysterious Vifram figure Sakurai bought to use as a character reference during development that nobody has been able to reliably match up to a character in-game but conspicuously would match up to something like Elma's Skell.
We just had GameExplain, who's Nintendo ties are quite known, post a curiously-timed video come out talking about Nintendo 1st-parties and how Spirits may not deconfirm. Everybody jumps to Waluigi, Geno and Isaac, but maybe it's referencing Elma instead?
There was talk recently that Hero was meant for basegame but held back for DLC because it would have been too expensive to pay royalties on every copy of the game because of the music, and the same situation is true for XCX, as Sony Music owns the rights to the soundtrack IIRC. But Sony also just participated in that Smash Tournament a week or two ago.
Elma also was in a prime position during SM4SH to be heavily considered for the DLC promo pick that Corrin ultimately came to be developed for. And just recently in the Elma thread, it was kind of revealed that Xenoblade Chronicles X as a series was cut from the list the same way those Rare games that were found early on were, indicating that XCX was supposed to be something and was removed for some reason.
Anybody with more knowledge (and time) please feel free to expand on and/or correct any misinformation, but otherwise, my own speculation is that with Elma's position timeline wise (XCX being released around the time of SM4SH's release so falling into the "before 2017" timeframe for being included), the Vifram figure, the secret Sawano music project, Elma's VA's NDA project, and the suspicious lack of basically anything XCX-related as well as it appearing like it was supposed to be there and then cut: Elma was intended for basegame but either cut entirely or held back for DLC like Hero because of the music royalties issue. Having her be DLC sidesteps the music royalties by only having a cut of the DLC be taken instead of the entire game. GameExplain's "spirits don't deconfirm!!1!" video is not a reference to someone like Waluigi or Isaac, but instead Elma, and while we keep getting a bunch of 3rd-parties deconfirmed, we're being left with a smaller and smaller pool to work with where a Nintendo 1st-party character seems more likely. And the supposed "Thank You Theory" (that every character in the pass was chosen because of their personal relevance to Sakurai or as a thank you to the company they originate from) also applies because Monolith Soft heavily contributed to the development of the game.
I think, if the final character is a Nintendo character, it's going to be Elma. It cannot be anybody else but Elma. The only thnigs going against her are Reggie's comments last VGA's about 'new worlds that haven't been seen' and the other characters all being 3rd-party (of which Elma, as a Nintendo character from the Xenoblade series that is currently in the game already as a Spirit, is seemingly triple-whammied by). But the GameExplain video is certainly curious nonetheless.