Thunderbolt is a big possibility. Actually I'm surprised Apple hasn't decide to use it yet now, in line with iPhone 4s launch last month since most Mac family have them. And you know Apple want them to be succeed or even surpassing USB 3.0
But the question is, does the iDevice with current dock connector can utilize Thunderbolt bandwidth, or we will stuck with USB 2 speed?
Or more importantly, will they change the design of the dock that has been there for years? I hope they don't and we just can get a Thunderbolt to dock connector.
I would be impressed if Apple used a connector that most Windows PCs (and quite a few Macs) don't have. Not to mention you lose compatibility with pretty much every car radio that supports iPod/iPhone (via USB).