Elma's really going to be the odd one out here
Pretty much all the other characters have been highly requested from fans, so to have Elma would be a bit of a wild card, and I think the best way to announce her would be to promote other Xenoblade stuff, like an X port for example
That being said, really anytime would be alright to announce her, and I mean, for those of us who are already Elma fans, there's really nothing that could make us any less hyped about her.
No matter when she's revealed, I'll be happy, and super excited that she's in
And also super relieved
Even if it means having to look at the datamines, assuming the game does get datamined
But they said that they'd reveal EVERY CHARACTER before release. meaning no secret characters, and I don't think we'll be getting a reveal so close to release this time.
Elma will almost certainly be revealed this month, or just at any random time before December
To me, October seems like a good time for another BIG Smash news dump. I feel we'll get a regular Direct for September, and then our last big Smash dump in October, possibly November.
Gamescom, the theoretical next regular Direct, or the theoretical next Smash Direct would be the perfect times to reveal her imo.