Question regarding the possible participants for the 13 and the 7, as someone who follows the lore but hasn't played any of the games so I might be fuzzy on the details:
Isn't aqua the only one who knows where within castle oblivion Ventus resides? A lot of debate seems to be whether aqua will be a light or a darkness, but if Ventus is definitely going to be a light, then wouldn't aqua have had to lead Sora to Ventus to get his heart back to him (since last I checked Sora still has ventus's heart), for him to participate in the war? Which would put her in the light column? I believe xemnas has been questioning Aqua's armor in the castle and trying to find Ven himself, but I am unfamiliar with how successful he's been with that. Maybe it's possible xemnas finds him with aqua's help but the lights somehow end up with him anyway?
I'm just speculating off the top of my head here but: Given Xehanort's deal lately is turning people into Norts, I'm not sure Ven not having his heart would be a dealbreaker for him. I could see him somehow getting the info from Fallen Aqua then heading over to Nort Ven and either succeeding (but then Team Light fixes him later somehow) or failing because Team Light catches up to them, drives them away, and saves Ven themselves.
There is LITERALLY a scene in one of the trailers where Vanitas stands over a defeated Aqua in Vens Throne room, surrounded by a Barrier. There is also a scene that shows Vanitas by Vens throne in another trailer.
It seems pretty obvious what happens here. Sora and or Riku and Mickey finally defeat Aqua, Sora and Aqua go to rescue Ven. Vanitas who is already there lies in wait, Aqua at the very least finds Ven, maybe awakens him. Either way immediately after Vanitas either knocks out Sora, and puts up the barrier, or just puts up the barrier so Sora cant get through after cheapshotting Aqua. Whether by hook or crook, he beats her, and stabs her in the chest releasing her heart and casting her back in RoD. Which is the scene we see at the very start of the new trailer.