There is one tricky part though with X music, its composed by Sawano, while XB2 has the same composers then the original.
I also would not be surprised if we would only get a new Xenoblade track of the original game like the Final Boss theme, Unfinished battle, Satorl Marsh(night version) or Tragic Decision
XCX having Sawano instead of Mitsuda, ACE, and the other composers is no different from how various other franchises also changed composers throughout their run.
Examples being Mario main games having music by Koji Kondo and the other composers at Nintendo's sound team, but the Mario & Luigi games being composed by Yoko Shimomura or Mario Tennis/Golf composed by Motoi Sakuraba.
Most of the series in Smash have so many games and had a lot of different composers working on them, so I don't think Xenoblade is going to be treated differently in this regard, especially considering the music in Ultimate is series based, so any other Xenoblade stage besides Gaur Plain would have the same music options.
Sawano did tweet earlier this year about a recording for a secret project, and it was later revealed to be alongside other known Smash composers
Do you have a source for that? Because I also heard from people that it was just for rerecording Uncontrollable for Elma in XB2
Full disclosure here: I was the one to first notice about this in a Discord server but it seems the rumor has spread.
Here's the thing: earlier this year, Sawano had a short recording session with a variety of instruments for an undisclosed project. Whatever it is, he's clearly under NDA.
Thing is, he's had various other projects this year and for all of them the recording sessions would last for longer and he usually schedules different kinds of instrumentation for different sessions, i.e. band and orchestra recorded separately. And for these projects, he'd record full soundtracks.
So, because this mysterious project took a lot less time, it can be inferred that it's less music as well, especially since he didn't do the usual separate sessions for different instruments but he recorded everything in one day.
Around this time, other composers also had short recording sessions that they later revealed to be for Smash. One such case I know it was Yoko Shimomura.
Basically, I follow a lot of composers on Twitter, I noticed some patterns and I postulated the hypothesis that Sawano
might be involved with Smash Bros. Ultimate. Keyword here being
might. I guess people took my theory elsewhere and it started to circulate. You know how things are.
One reason behind my theory is the fact that a lot of composers in Smash often have been involved in games represented in Smash, sometimes even reworking their own songs for the occasion. Yoko Shimomura, for example, has revisited her work from the Mario & Luigi games and Street Fighter II and she's also been involved in the Xenoblade and Final Fantasy series, as she composed music for XC1 and FFXV. 3 of the composers in Ultimate have previously worked on Metal Gear Solid games, as another example. Jun Senoue and Tomoya Ohtani, known for their work in the Sonic series, are working on Ultimate with Senoue also having worked on Brawl.
In other words, the timing of Sawano's recording session, it being unusually short for his usual projects, him being involved with Xenoblade X which a game with potential representation in Ultimate and the strong precedence for Smash to bring back composers that were involved in the source games it draws from are the reasoning behind why I think there's a possibility that Sawano could be involved in this game, but it's by no means a guarantee.
This went longer than I expected, but I hope I've clarified everything.