Duma is an interesting choice there. Better than another alt for someone with a bunch of them already, and hopefully we see more characters like that down the track too.
I think these characters, who all also have votes available for CYL, are all slam dunks.
-Mila, obviously. She'll probably be Astra to compliment Duma's Anima.
-Naga. It'd own if they mixed her Awakening design with her FE4 dragon depiction. Light hero?
-Yune. Actual Yune, not just Micaiah in her Light Priestess outfit. Dark Hero, probably?
-Ashera. Also light, probably.
-Athos. The living legend, one of the big heroes of Elibe. Easy Anima.
-Anankos. Again could be anything. Has a human design to go with his dragon form.
-Ashunera. I dunno if they'd go with Yune, Ashera AND Ashunera in one go, though. Maybe for Super Mythically Legendary Heroes next year.
-Hel. She's literally a goddess! She is absolutely going to the Mythic pool if her daughter Eir is here.
Then we have god-adjacent characters we can go with like:
-Dheginsea. He's part of that big picture with Altina & Soan, incidentally
-Nagi. I'm not sure they'd ever waste a Mythic slot on Nagi
of all people, though.
-Medeus. Earth Dragons are shown as a big threat, I think he's about on the level of Duma & Mila even if he's not worshipped as a god. I mean you need like 3 different things to finally seal him away. Twice. IMO they could get away with Earth Dragon Medeus being a Legendary Hero and Shadow Dragon Medeus being a Mythic.
-Lilith. Daughter of Anankos, so she could get in like Eir could I suppose.
& I'm sure there's some others I'm forgetting. They could, sigh, also go with another Corrin alt, Zelgius alt, things like that. I guess now's a good a time to mention that "Grima", as titled "Grima", is in the CYL. So they could probably sneak in another Grima that way and have him act as a manakete that transforms into "The Creation" from SOV.
After that, the game does have a number of other god-like characters but idk if they're really eligible:
-Elibe has another 3 of its Legends. Roland, Bramimond, Durban. You couldn't vote for any of them, though, so I'm not sure they'll go with any of them?
-Speaking of, did you know that Elimine, another of the Elibe Legends, does have a canon appearence? She shows up in, of all things, the Double Dash Demo Disc that let you connect to FE7 and get a bunch of gifts. Still couldn't vote for her, though.
-Altina, as noted before. It's a crying shame she's otherwise ignored.
-Soan, the lion laguz there, also counts. But he has even less going on than Altina, sadly.
-Then there's a bunch of Crusaders and Legends that we don't have any real information on in terms of who they were, their apeparences and so on.