I can imagine Rex having a shot if there's a second season of DLC somewhere down the line (if not, he'll most likely be in a future entry), which is why I wouldn't count him out. I mean, Sakurai works in mysterious ways. However, I'm very content that the XC2 got as much content as it did, considering the development cycle.
Also, I think Nonno summarized the appeal to Elma fairly well. She's a caring, humble, courageous leader and a strong independent person that saved humanity from total annihilation. She's also a dual-wielding gun/sword user that pilots a giant flying robot that turns into a ground vehicle. Moreover, she's actually a robot and an alien simultaneously and the deuteragonist of her game. If I knew nothing about her, I'd still main her because she's cool as heck, and there's nothing like her in Smash so far. There's too much to like. I'm also glad that he mentioned that Elma's a woman of color (a form she chose, to boot) because I can only think of like... 4 more from Nintendo games, and they have way less of a chance than Elma. Women in general have gotten the short end of the stick in Smash (4 started correcting this), but women of color have no stick at all, as far as reps go. Not even palette swaps. There's nothing wrong with a little diversity, and it justifiably appeals to some people (myself included). It isn't the only reason to want her in, but the point is that she's plain ol' fascinating for a ton of reasons, even if you're unfamiliar with the game she comes from. In short, her reception will be positive overall. Of course there'll be the "But my character!" complainers here and there, but that is the way of the world.
Anyway, I hope if she's added as DLC, she'll get revealed early, just so I can play her lol. However, I'm thinking she might be revealed last like Shulk, just because... or at least close to last.