As far as the admirals go, they had a very specific character theme with Aokiji (Blue Pheasant), Kizaru (Yellow Monkey), and Akainu (Red Dog). I'm completely unfamiliar with the myth the theme is based off of, so I don't know if any other replacements could be brought in this way.

If, however, the titles of Aokiji and Akainu were passed onto new admirals, that gives us a better idea of what might be possible.

Garp has a dog theme to his appearance anyway, and he might just be the one to inherit the Mera Mera Fruit if possible, especially given his relationship to Ace. This would fit the naming theme and the logia theme of the admirals. I don't think it's likely based on the fact that Sakazuki is supposably going for an even darker administration than last time, though it could be that Sakazuki wants to keep a closer eye on Garp and try to force him into a position where he has less leeway to do as he pleases. If this pattern is followed, then I suspect that the new Aokiji will be someone completely new, possibly with the long-speculater Water Logia.

If, however, the original theme regarding the Admirals is no longer in effect by Oda, that opens up a few new possibilities. I doubt any of the side Vice-Admirals with barely any appearance like Cancer, Momonga, Onigumo, etc. would really work. They just don't matter enough.

As a dark horse, or leopard in this case, what about Rob Lucci? We don't know what happened to him post-timeskip, his strength and track record are still well above the majority of the marines, his loyalty and commitment to the "dark justice" makes him an ideal subordinate under Sakazuki's administration, and he has several loyal followers already.

On a related note, the Shichibukai...

Until we get confirmation, we have quite the roster change:

People who are confirmed Shichibukai:
Doflamingo - Confirmed to retain his shichibukai status.
Mihawk - Can't find confirmation that he's retained his shichibukai status, but I also don't see any reason why he wouldn't still be a shichibukai.
Trafalgar Law - Confirmed to be a new shichibukai.

People who might no longer have the shichibukai status (or lose it shortly)
Boa Hancock - She's aided Luffy quite a bit. Supposably she's been hiding it enough that the government hasn't caught on and revoked her status, but it seems less likely each time that she hasn't been found out yet. That could potentially get her kicked out.
Bartholomew Kuma - If Bartholomew Kuma truly has lost all of his free will, it sees pointless to keep him as a shichibukai when he's just as loyal to the world government either way. The point of the shichibukai is for the World Government to forge uneasy truces with the more dangerous of pirates to minimize conflict on both sides and to team up agains threats like the Yonkou. Better to fill that slot with an actual pirate who'd be a threat otherwise.

People who are definitely no longer shichibukai:
Blackbeard - Confirmed to not retain his shichibukai status.
Jinbe - Confirmed to not retain his shichibukai status.
Gekko Moria - Confirmed to not retain his shichibukai status. Supposably he was killed on the battlefield, but he could also very well be locked away somewhere secretly as well.

So we have anywhere from three to five new guys, one of which is Trafalgar Law. Speculation as to who could be the replacements?

Buggy the Clown - There's already been a lot of discussion to this and hints in the story, specifically the letter he received just before the timeskip. Mostly due to the fact that his new crew is probably far more dangerous than Buggy himself.

Crocodile - The government might consider reestablishing his shichibukai status out of desperation now that he's out. He would be a valuable ally in the current climate. He's not exactly trustworthy, but look at the others.

As for the remaining Supernovas... Eustass Kid, Scatchman Apoo, and Basil Hawkins have been shown, and considering their alliance talks it seems likely that any of them have become shichibukai.

X Drake was mentioned as having made a name for himself, but given his anti-marine demeanor, he seems unlikely. Bonney's definitely out after her encounter with Blackbeard.

Capone Bege and Urouge haven't been mentioned post-timeskip yet. I don't see them as likely choices, but it's worth considering.