Alright, so Ven's heart is "sleeping" within a place "between light and darkness." The game's premise is set within a "dream," where Sora and Riku have to travel through incomplete worlds
lying in stasis "between light and darkness". I think you can see where I'm going with this.
To elaborate, Ventus' heart is somewhere deep inside the Dreamworld, most likely beyond the Keyhole of Sleep. However, because Ventus is presently inside Sora's heart, I'm going to say that this will only take place in Sora's story, since Riku shouldn't have any bonds/affiliations with him. It's stated by Yen Sid that Ven will need his friends (Aqua, Terra, and Mickey) to help him wake up. Unfortunately, Aqua is out of commission, Terra is having
his own problems, and Mickey can't really do much of anything (I'd imagine it'd be very complicated, and painful, to separate a person from their heart, especially one with multiple hearts) This leaves the task to Sora, who will have to look within himself, aka go inside his dreams, and become the "friend" that Ventus needs to guide him back to the realm of light.
To add to my previous theory, I'm also going to assume that Terra, whether in person or his heart, is also going to show up in Riku's side of the story. With Young Xehanort and Vanitas showing up, the possibility of my first theory and Terra now being free from MX should be very plausible.
No idea about Aqua though.