I think Elma has a decent shot at getting into the game, but I also think her chances are overstated by many. I apologize in advance to
Rie Sonomura
First of all, the XCX composer thing. Given Ultimate's time frame and the project plan, it seems very likely that there will be XCX content in this game in some form. Getting some music from the game seems like a no brainer. I think it's entirely possible we could get a new original piece of music from XCX with no new character. There have been similar things done in the past, such as The Mysterious Murasame Castle getting it's own piece of original music in Smash 4 despite no character or stage. Really, the thing with the XCX composer only solidifies that we'll get new music for XCX which could potentially be added onto a new XCX stage or maybe another Xenoblade stage not related to X.
Then there's the Xenoblade characters thing from Bill Trinen. If you go back and watch the interview, it's clear that Bill has no idea what he's talking about. He just had a real brain fart moment and was trying to think of what franchises were new to Smash and spouted out Xenoblade when he couldn't even remember what characters from it were in Smash. Plus, if he said "The Xenoblade Character" in reference to Shulk that would've made him look like an idiot.
And then you DO have to take into account that if Elma gets into the game, it won't be because of fan demand. XCX was released after the ballot in the west and in the midst of the ballot in Japan. There's no way she was able to build up enough support at the time to be considered for fan demand, so her only chance at getting in is if Sakurai picks her himself, though that's still entirely possible.
I stated this at the beginning, and I'll say it again. I think Elma has a decent shot at getting into the game, but people make her out out to be way more likely than I think she actually is. Maybe she'll be revealed at the Direct on Wednesday and everything I said up above will get thrown out the window but I wouldn't hold my breathe for it.