I could see 2 more Xenoblade reps, but if I had to guess, I'd say Rex makes it in before her.
But why though? Xenoblade 2 just came out. The fact that the Inkling's design is based on the original Splatoon (and not Splatoon 2) potentially rules out Rex and Pyra.
I think from what we know, a rep from XCX is far more likely than XC2. If Rex and Pyra/Mythra aren't in the base game, they're definitely strong candidates for DLC.