First off, even though I expect most characters (like the students) to show up as NPCs in the story, I don't think everybody will be playable since there are just too many characters and some of them would play too similar to each other (unless there are clones like in the first Warriors game, I guess) and also there are some characters who are way more popular than others, so they would take priority.
With that said, these are my predictions:
Avatar Character
– Byleth (female default, male alt)
Black Eagles
– Edelgard (confirmed)
– Hubert (he is shown in the foreground alongside Edelgard and got a new design like the Lords, in addition to playing an important role as Edelgard's second in command, so of course he will be playable)
– Bernadetta (as much as it pains me to admit it, she is clearly the most popular Black Eagles student besides Edelgard. I mean, she just got third place once again in CYL6, so I definitely expect her to be playable)
– Dorothea/Petra (if we get someone else besides Bernadetta, it will be either Dorothea or Petra. The former is the more popular of the two, judging by the fact that she ranked higher in CYL6, but the latter would be easier to translate into gameplay since she's a warrior, not a singer. I just want Petra dammit! I want her with Lyn's moveset!)
Blue Lions
– Dimitri (confirmed)
– Dedue (I'm not entirely sure about him to be honest, since he's not very popular compared to other characters, but I guess it would feel wrong to have Hubert and Hilda but not Dedue for some reason)
– Felix (he is by far the most popular male character in the game after Dimitri and Claude. He just got 3rd place like Bernadetta in CYL6, so of course he'll be playable here)
– Ingrid (Yeah, that's right, Ingrid. She may not be one of the most popular female characters in the game, but she's the only canon pegasus knight, so I'm sure she'll be playable. Also I doubt they would make Blue Lions male only)
Golden Deer
– Claude (confirmed)
– Hilda (we can see her protecting Claude from Dimitri's attack in the trailer. If Hubert is playable, she will be too)
– Lysithea (most popular female character in the game besides Edelgard, perhaps rivaled only by Marianne. She literally won CYL alongside the Lords when Three Houses came out, so of course she will be playable)
– Marianne (most popular female character besides Edelgard and Lysithea, she literally won the next CYL after them. I'm sure she will be playable. She may be also the reason why we may get four students per house instead of three: they most likely want to make both Lysithea and Marianne playable, but it would feel wrong for the Golden Deer to have more playable characters than everybody else)
Church/Misc.
– Seteth (he's like the second-in-command of the Church and the "Lord" of the route where you reject Edelgard, as well as being the only canon wyvern rider other than Claude, whose moveset would be very different from his. We need some flyers, so he gets in)
– Catherine (she is like the strongest fighter in the Church. She is made for this game, so she should be playable)
– Shamir (she's cool. If Catherine is playable, so will she)
– Gatekeeper (you know he is coming. The madman literally won the next CYL after the Lords. He deserves to become the badass he was always meant to be!)
– Jeritza (the Death Knight would feel right at home in this game. He would be the Lu Bu equivalent)
Well, that's it. 18 characters, just like Hyrule Warriors: Age of Calamity. Since this game is somewhat similar to it, I guess the initial roster would be too.
We basically get a full squad of 4 characters per House/Church + Byleth and Jeritza as the badass lone wolf.
The characters that I haven't mentioned still could show up as non-playable NPCs, just like Judith and Randolph, who we can see in the trailer fighting against Edelgard and Dimitri. Also we can see Rhea as The Inmaculate One. Hopefully she will be playable in her human form (I want to punch people as her), but she might just be a boss fight or a huge character for a special mode like Ganon in Hyrule Warriors.
Jeralt will probably die again lol.
Monica/Kronya and the purple-haired OC will just be enemy NPCs.
No Anna or Ashen Wolves. I don't feel they are very popular in this game. Perhaps as DLC again?