I have a really stupid Smash theory, that may back up Rex a bit; but maybe I'm overthinking things quite a bit. It's probably been brought up here before, but I don't usually go on here often.
So there are friend icons in things like Arena right? Most of these icons got added in the base game, however each DLC fighter gets their own icon added when they get released. There has been no fighter released, that already had an icon in base game. Though granted the likelihood of them having an icon originally was low anyways (with the exception of Min Min, who also lacked an icon; but I think only the Spring Man render for the AT got one for ARMS). If we go through the list, every fighter, every AT, and a notable list of "exceptions" form the icons.
Friend icons are images depicting characters or symbols which identify players in the online modes of Super Smash Bros. Brawl, Super Smash Bros. 4, and Super Smash Bros. Ultimate. Some of them are available from the start, while others are unlocked...
www.ssbwiki.com
You'll see characters like Bandana Dee, Geno, Dixie Kong, Shantae characters that people WANT in the game as some of these "exceptions" right (though there are a lot of filler characters in there too). But if you scroll down to the Xenoblade section, it's missing Rex. Sure it's also missing Elma, Melia, and the like; but the important thing to note is that it HAS both Pyra and Mythra but no Rex in it. Rex is obviously a character that Sakurai has wanted, and he even got a bonus costume added right towards the end.
So what if the reason there is no Rex in the icons, is because they left it open for a potential season 2? They could then add Rex and use his render as the icon?
Again take this with a huge grain of salt, but I just feel like it's too odd to just be a coincidence (especially with Pyra and Mythra there). The entire thing started out as a really stupid idea, but maybe there's a chance that this backs him up a bit.