Right now I would say that Aqua is the easiest to say. All that is needed is to find a way into the realm of Darkness.
Well, considering that the Realm of Darkness isn't small (it
is a realm, after all), and she's been trying to find her way out-- actually, I don't remember why she's in there...
But I would say that she is not the easiest to get to. She's in an entire realm seperate from Sora, and then when they get to her, how are they going to get out? One does not simply WALK out of the Darkness. =_=
For Ventus a.k.a Ven it is a matter of restoring his heart and then Sora giving it back to him.
They have to find him first. Axel spent a poo load of time looking for the Chamber of Waking (whichever one Ven was in), and even if he did find it, he's currently dead in the series.
The most productive (and least boring) way for Sora and Co. to find Ven is to first find Aqua first, since she placed him in the chamber and altered the Land of Departure.
Terra is where it is tricky. Supposedly both Terra's Heart and Body have been consumed by darkness. His soul I believe still lingers in the Lingering Sentiment. I believe that both Terra's Heart and Body are in the Realm of Darkness and can be brought together.
Well, before he became Xehanort (Ansem the Wise's apprentice), he was Terranort. Then his Heartless was destroyed, then his Nobody, so I'd assume that the creature that comes back to life is Terranort, not Terra and Master Xehanort seperately.
I'm pretty sure this would be Sora's main objective in the next game (or the main antagonist, also considering it's the Xehanort Saga/Chronicles). Terranort wouldn't just sit back for Sora and Co. to find Aqua (I can actually see it being a separate scenario in which Sora and Aqua have separate scenarios), so we should assume that saving Terra would be high on the priority list for the next main installment.
Also, in regards to the Lingering Sentiment, Terra does not have his complete soul. The Lingering Sentiment is Terra's armor with some of his soul in it, which is why it can move (a.k.a. "be animate"). If the two (Terra and Master Xehanort) were to come back, then Terra would have an incomplete soul, and Xehanort could just kill him off then.
Of course this theory has some holes because MX's heart was in Terra's body and supposedly they are still fighting for control. It's quite possible that you could end up fighting Xehanort in his true body in the next game if the same were to apply to him as to Terra.
A possibility, I guess.
Tatsu said:
^Right now this is just wishful thinking. This adventure is not happening. Apparently Sora's story is cancelled! KH is now about Data Sora and the adventures in the Data World.
The Data Sora's story is to make what happened eleven years ago ("Birth by Sleep") relevant to the real Sora. Without Coded, Sora and Co. would know nothing about people in the past that they don't necessarily know about, or at least until they catch wind that Xehanort is back. They wouldn't even think that he's in need of help (seeing as he's actually Terranort, and not Apprentice Xehanort).
Coded isn't a cancellation of Sora's story. It's what's keeping Sora's story flowing.