Your arguments seem lacking. Because X has less content you assume it's more likely to get in. Let's just get this out there, how many or few tracks it has almost certainly doesn't actually matter. There's still plenty to pull plus remixes. That's kind of a moot point. Min Min proved that spirits can be reused for the packs too, so preexisting ones shouldn't really affect it.
Also, if Elma was going to be in it...why isn't she? Surely wasn't licensing issues. Moveset likely wouldn't be that complex. If she was late, why was a plant prioritized? Why wasn't she in pass one? Unlike Rex, she doesn't have the timing excuse as her game was well out in time. Rex's likeliness aside, Elma seems highly unlikely as she had her chance...and nothing. She also kind of lacks the fan backing to push her back into the spotlight. In regards to the series as a whole, Rex also seems more likely as Elma as it stands is just kind of a weird, experimental spin off. Plus, don't forget how much they love their hype. Either one may only appeal to smaller audiences, but Rex would appeal to more.
Are either likely? At this point I don't know, but if we're getting one, it's probably going to be Rex.