I would think Ventus. Currently, Terra is with A Hornet X so he can't be saved yet, and Mickey said in the RE:coded Secret Ending,So i would say Ventus.Spoiler ShowThat they have located Ventus.
We don't know if Terra is really still with Tan Hero X, given all the implications towards Riku and the like.
Actually Mickey says:
Spoiler Show"Yen Sid, I think we're finally close to find out where Ven's heart is.
That implies they're not completely sure yet, but close to finding out.
It all depends on what is actually covered in 3D besides the MoM-exam.
But now when I consider Mickey's statement above I could imagine one task in 3D for Riku and Sora maybe finding out if Mickey's suspicions of Ven's heart are right and to somehow get in contact with that heart to get to know in what state it currently is. I.e. if and when yes how much it has recovered etc.
And there's only two real options for Van being independant of Ven:
Vanitas returning
Ven's heart using some of Sora's darkness to repair his heart. Which may result in Vanita's return inadverdantly, anyway.
fixed that minor letter error, lol, otherwise it would make no sense.
That's actually a neat idea.
Sora is closely connected with Ven, while Ven is still connected with Vanitas (if he still exists, that is).
So it would be not impossible for Vanitas to us his connection to Ven to jump over to Sora over the Sora/Ven-connection and hide inside Sora's darkness, waiting for his turn to strike.
Of course it would be inadvertantly, but add a real dramatic twist to the story. Just being saved Ven would be in immediate danger again, as would Sora and co.