In my mind, it depends of when they decided to do this second season pass.
If it was all the way back to the end of development of the base game that they planned to do two season passes (but revealed two just now), then Rex is out, as there would be no reason for them to make his mii costume if they were going to do so many DLC chars, and also the fact that xenoblade 2 was not fully ready by that point.
Now, if they decided to do the second season pass more recently (more likely), then Rex has a big high chance of being on it, heck, given how Sakurai talked about him, I would be as far as to say Rex is one of the safest shoes in. Elma? Well, if this thing they decided to do for Xenoblade 1 is the xenoblade project they were working on, then xenoblade X2 is really not happening any time soon, possibly, not even on switch lifespan. So unless they also intend to bring X to the switch (with the money they will get from the thing they are doing for 1), Elma has no chance anymore for ultimate.
And honestly, the combo of Rex as a figher plus Elma as mii costume would do no favours for X, it would only send a indirect message of it being less than the numbered titles (as if such message was not being sent enough already). Rather they leave X alone then, and give a problem representation on the next smash, which will probably be around X2 release, by the way things are going.
I was thinking about this Xeno 1 remaster and I think I decided what I will do. Wait for it to release, and ask for spoilers of it having something added that hint at X in any shape, being acknowleding X as part of the multiverse, or that the conduit may have gone to X universe, or even a non canonical hint of X being ported or about X2. A special feature like the land of challenge and someone from X showing up, would be okay too. But if there is nothing there, no mention of X whatsoever, even if indirect, and only the retcons to make 1 fits with 2 better, well, I will give it a hard pass.