Aokiji's appearance in the manga was really a turning point for the whole story, and I'm eager to see when the next one of the Admirals will show up. So, the focus of the thread is basically when you think the next admiral will show and which one it will be.
Personally, after Thriller Bark is done I would like to see Akainu show up. With Moria's defeat and Enies Lobby's destruction I'd think the World Government would send one of their powerhouses to deal with the Strawhats before they become to big of a problem to handle (like they are now with Kuma). From what little we saw of Akainu he sure seems like a coldhearted bastard that would pursue his prey relentlessly with the whole 'Absolute Justice' mentality.
So in short I think Akainu will appear after Thriller Bark has ended and be sent to deal with them, especially if Kuma fails in doing so (and he probably will).