I could not be more pumped for Birth by Sleep Vol. 2. Here's what I'm hoping to see:
We still get to play with the same three characters in BbS, only with a twist. Terra's story would be as Xehanort, meaning we would have the option of playing as Xehanort, in Radiant Garden, doing work for Ansem the Wise, traveling the worlds to help him conduct studies, and eventually conducting the experiments with the other lab assistants that create the Emblem Heartless.
Aqua's story would take place in the Realm of Darkness and discover the scourge of the Heartless being drawn into the world by Xehanort, and somehow Mickey would learn this (as well as Sora's ties to everyone) from her, hence his departure and letter to Donald in KH.
As for Ven, my best guess is he remains in Castle Oblivion. Remember, in 358/2 Days, Axel was ordered to locate the Chamber of Waking, so I would assume somewhere around KH2 Ven's still in there. Maybe his journey would have to do more with memories and the Realm of Light.
Since Vanitas is no more, I would guess this is when Unversed stop popping up and the Heartless make their debut. Makes sense, considering Terra-Xehanort and Aqua's probable storylines.
I suspect during Terra-Xehanort's storyline, more will be learned about Lea and Isa. After all, there must have been some reason they not only got consumed by the darkness, but retained their human forms as Nobodies.
Speaking of which, hopefully we'll learn the human forms of the other Organization members. I know they didn't necessarily all turn into Nobodies at once, but they've got to have SOME background worth knowing about.
The formation of End of the World and The World That Never Was. Somehow I suspect they're connected to the influx of darkness that occurred after Xehanort's experiments.
The formation of Traverse Town, which I believe was created after so many worlds succumbed to the darkness.
How Xehanort sent Kairi off to Destiny Islands, leading to his discovery of Sora at the top of KH.
Whew! That's a lot of hoping!
Here's what I suspect will happen timing-wise: 3D will be released towards the end of 2011, BbS Vol. 2 will come out mid-2012, and KH3 will come out in 2013. Given what we can guess about KH3 already, it's fair to say BbS2 will be the last background we need before Sora and Riku take on Xehanort's final incarnation.
Lemme know if you think I'm on the money, wildly off, overthinking this, not thinking this enough, if I left anything important out, etc. I'm just really excited for these next few games in the series!