Okay, so i just finished aqua's story in BBS and Yen sid tells aqua ven's heart is sleeping so i'm thinking maybe toward the end of 3D sora will find himself at castle oblivion cause isn't that a world in sleep? i don't know so anyway maybe ven's heart in sora will lead him to ven and then he gives ven his heart back. Then sora, ven, and riku get back to yen sid. Then go looking for aqua. This is my theory so tell me what you think.

This would defeat the purpose of Aqua being the only one who knows where Ven is. Also, Castle Oblivion isn't in the realm of sleep.

The only theory I have is Sora and Riku getting Xehanort's grip on Terra, which will then have Terra make a goal of finding Aqua and Ventus.

i think I know what your getting at. that ven's heart is sleeping and it can only be found in a deep sleep. As far as Aqua only knowing where Ven is could only apply to his physical body which is in castle oblivion.