I was just taking a look at what dragons haven't been on the Halloween banner yet and here is what I have come up with:
Heroes: Nifl, Muspell, Askr, Embla and Ymir
Shadow Dragon: Medeus, Gotoh, Bantu, Brzark, Xemcel, Khozen and Nagi
Binding Blade: Ein, Jahn, Fae and Idunn
Awakening: Nah and A!Tiki
Fates: M!Kana, F!Kana, Lilith, Garon and Anankos
Engage: M!Alear, F!Alear, Lumera, Veyle, Nel, Rafal, Zelestia/Zephia and Sombron
Radiant Dawn: Nasir and Gareth
Three Houses: Flayn and Seteth
Tokyo Mirage Sessions #FE: Tiki
Fire Emblem Warriors: Velezark
If I missed anyone let me know and I will add them to the list.
As to who I think is likely I think the Kanas and probably M!Alear are fairly likely. I really feel like it will be all dragons again since we still have a bunch to go and that has been the theme for several years now. I was just trying to get an idea of who has a shot and they aren't running out of dragons anytime soon.