I have a theory. I think the base roster of Super Smash Bros Ultimate is going to focus on older characters, and that newer characters will be early DLC. DLC that's been planned during development.
I think the base game is going to focus on including characters like Gardevoir, Gothitelle, and Elma, and DLC will include characters like Rex and an Alolan Pokemon, along with eventually Arms and Fire Emblem: The Three Houses.
It's not that crazy that an Alolan Pokemon might be reserved for very early (almost immediate) DLC. Super Smash Bros Melee originally didn't even plan on including a Gen II Pokemon, but Pichu luckily made it in as a result of echo fighters being included as extras. Plus Pokemon Omega Ruby/Alpha Sapphire and Xenoblade X were really popular games among the fanbase, so it would be a good strategy to include as many fan favorites as possible if you start with the earlier characters and then include the newer fighters after release. This is Super Smash Bros Ultimate after all. So it makes it sense to include Gardevoir, Gothitelle, and Elma to ensure every Pokemon and Xenoblade game has a fighter.
The fact that we haven't seen an Alolan Pokemon stage yet may support this too, as it would likely be packaged with the Alolan Pokemon DLC and a Xenoblade 2 stage packaged with Rex, as well as a Fire Emblem 16 stage packaged with Byleth, and so on and so on.
I also feel like we may have seen an Elma assist trophy by now if she wasn't intended to be a playable fighter. Of course, just because we haven't seen something yet, doesn't mean it's not gonna happen.
In other words, if you plan on doing DLC in advance, the best strategy would be to include the long requested fan favorites first, then include the most recent ones. Though, I don't think DLC is going to be paid this time though, seeing as it would contradict the "Everyone is Here" selling point if you had to purchase fighters.