I join the game of predictions. What I think would make sense to be announced:
- Superman. It seems inevitable for several reasons (apart from Gunn having said he's working on it).
On paper, he's the most iconic superhero. But currently, he's also a somewhat withered icon who needs a serious comeback, especially when he's supposed to be THE pillar of the universe that you're trying to set up.
An official announcement of a new movie with a new actor would bring the character back into the spotlight and create some interest. But it would also be a clear signal that the DC Universe in film is finally moving forward, abandoning for good what was started with Cavill in the role (who, I maintain, is no longer as popular as some "internet echo chambers" would believe, including this forum).
- Green Lantern. Of the more iconic DC characters, he's only had one film adaptation so far, already a decade ago, so there's a bit more freshness here, which is always welcome. He's also a character who, by nature, opens up the DC world wide by making the connection between Earth and all the space stuff. It would make sense to use him from the get go.
Because representation is a particularly scrutinized component in Hollywood, I think the character of John Stewart will be chosen under the mask (or... in the ring ?). While not the "fan favorite" version, I believe he remains the most popular with the general public, thanks to the Justice League animated series. WB might consider this an easier sell...
- Then I think we'll have a new female figurehead for that new universe. No Wonder Woman whose success, even if it has faded a little, is too recent to be reintroduced, nor Harley Quinn, who's been very prominent in recent years and already waiting for a new adaptation with Lady Gaga.
The character that would make sense to me is Zatanna. She's not well known by the general public but still benefits from a solid an base, which can boost the hype through social medias, etc. Besides, she's not your typical "woman in a combat suit" like Captain Marvel, Catwoman or Black Widow. She's a magician and so, there's a novelty to play on here, which could allow the character to stand out a bit.
Last thing, after the superheroic and science fiction bases introduces by the two first films, Zatanna could smoothly bring one last rule to this new cinematic universe with magic...
All that being said, I admit I don't really expect Gunn to bring her on as I can imagine him favoring a more obscure female character here instead.
And that's it !
Maybe Gunn will add another lesser known character to the mix but I don't think the studio will (nor should) announce more than 3 or 4 films. What works in this plan, for me, is that I believe it makes strategic sense on several levels.
First, there's diversity in the characters. In their popularity, their representation. But above everything, they all have quite distinct vibes. This would give a certain creative force to the project by allowing very different types of films and avoid immediately becoming slave to a formula.
Then, at the level of the diegesis which is the most important aspect, like said, the foundations and rules of the universe would immediately be laid down. You could tell pretty much any type of story from there.
And last thing, while these films having some complementarities, they wouldn't need to be, right away, too interconnected story-wise. If there's a thing WB knows too well now, it's that it's better not to put all the eggs in one basket too quickly... This would leaves them a certain freedom "just in case".
Anyway, that's my take on it. We'll see !