I don't think Braig would be the next 'main' villain, he isn't powerful enough.
Uh, Braig is almost the final boss for Aqua. Give him a bit more darkness and I'm sure he could easily be bumped up to Main Villain Power Status. Especially if he gets a hold of KH in some way.
Braig isn't Main Villain material he's ambitious but lacks the necessary foresight to see grand plans through.
Uh, how do you even know that when we've never even seen him -try- to perform a "grand plan". He seemed ambitious enough to lead Xehanort and the others into creating the Heartless, and, by extension, thus creating the Organization. I don't think you give him enough credit.
Plus, it seems obvious to me that Nomura has been slowly building up Braig/Xigbar's importance over the games. In KH2 he got a throwaway line about having seen more Keyblade Wielders, in Days we see him poking fun at Roxas about his "future" and all that while he sees Ventus in Xion, then in BBS, he is an important character in not one, but all three character's scenarios more or less, and almost a final boss for one of them. Then he proceeds to created the Emblem Heartless among other things.
I say there's more to Xigbar then meets the eye, and I could easily see him, and perhaps the complete forms of the other apprentices, being the new main villain going forward, at least at first.
I think that's a whole lot better then Nomura suddenly pulling a brand new villain out of nowhere and pretending like he was a secret threat all along.