He can't use Sora and Ven, he already talked about how his own Pure Light vs Pure Dark faield, he needs ALL The pieces (hence why he needs the 13 darknesses and the 7 lights)
Sora's keyblade and Ven's keyblade are different keyblades. This is why he can wield two. He has his own, and Ventus'. This was shown in the KH Ultimania. So that has nothing to do with the Kingdom Key
Terra hasn't regained anything, he's still one of those 13 vessels.
Ven will wake up yeah.
As for Aqua, the world of Darkness is huge, she could still just be aimlessly wandering. Still alive though, since in KH2 Final Mix Terra-Xehanort was seen conversing with her armor.
Well guess time will tell in KH3 then.
Uhm Seven Light versus 13 Darkness, let's see
So we have Sora, Riku, Lea ( Axel true form) Mickey, Ven ( Since he's most likely to be aweken) Kairi ( Such a lame choice for a warrior consider she have zero to none combat ability) Aqua ( probably escape the world of Darkness) as 7 Light
And 12 other Xehanort Clones/Version and a mysterious 13th person as 13 Darkness
This is probably my own theory, but how about another game before we actually have KH3:
It will be about Aqua quest to escape the world of darkness, that places is HUGE, she would meet a mysterious Dude/Woman who lost his/her memories and trap in this world, he/she will have some sort of super power. Aqua and that person work their way out of the world of darkness, by some way of battling the monster in there, and in the end, it's reveal that this person is the 13th Darkness, who lost his memories or have been seal away, but then regain his/her memories or most of his/her power, and is recuit by Xehanort. Mean while, Aqua may have develope a love interest with him, and will join the 7 Light to fight and free him, since she saw some good/light in him yadayada romantic crap. And all of this happen at the same time as Dream Drop Distance
What do you think? is this a good story for KH Vita?
Hmm, not sure aqua has the thing about be shy and she doesn't look like lover type of person but friend person at that. And if she did that would be awesome story then. How about calling it Kingdom Hearts Vita just how you said it was KH Vita then? It sounds like good name for side quest game that’s leads to KH3 and one more side game won’t hurt series. There should use the battle system from birth by sleep plus 356/2 days in it too but with some change to it like when HP drops below 40% that user can use his/hers limit break or use item called aura to use their limit break.
As for keyblade’s aqua can only use Masters Keyblade until she gets out of the darkness place and finds her armor and keyblade. But this 13 darkness dude beings aqua her armor and keyblade back for her because there had no more need for it or something like that. But their place darkness in the armor and aqua has to battle her armor form to gain back her keyblade. The rest I’m not sure on but I have some idea’s what will happen next maybe but it’s better to see how it plays out. If I had to write storyline I would know good way but I would need help to make sure it stays true to itself and how it plays out.