I'll post this here as the thread seems slightly more active then the Itsuki one...
The ARMS addition makes me... mildly optimistic about TMS#FE chances? Like, TMS#FE does completely lack music representation, in spite of having a spirit. It also hasn't had a spirit event to promote its Switch port (not yet at least, as Astral Chain proves those can come anytime for recent Switch titles). It'd be easy to add a stage for it... If Tsubasa was the character, then they could just upgrade her spirit and make the spirit board contain the rest of the party who didn't get spirits in the base game. If Itsuki was the character, then you can still fill a spirit board with the rest of the characters.
Now, there is the fact that Sakurai acknowledged FE has too many characters, but this would be... different enough, I think? Tsubasa possibly getting in as a lance user would make her instantly stand out from the crowd, and that's ignoring the fact that she can also summon a giant ghostly Caeda to fight with her.
Semi-relevant, but I find it funny that Itsuki was listed as one of the heroes in the
8-bit Fire Emblem 30th anniversary video, but Tsubasa is the one getting into the Fire Emblem Heroes 30th
anniversary banner. It's obviously partly for the sake of balancing the colours, but it still amuses me that not even FEH can consistently decide which of them is the protagonist.
Obviously I don't think Tsubasa getting one of the six precious character slots would be good for the salt-levels of the community, but, until a spirit event happens, I honestly feel like it's... kind of likely?