Coby's the obvious only guarantee
There's a few things to consider after that. First thing being, how many posts will need to be filled? Sakazuki's an obvious gonner. Kizaru, I would guess, is going down with him, although to be honest I've never really felt him as a particularly bad Marine. Fujitora and Ryokugyu though, they should stick about given their (implied in Ryokugyu's case) anti-WG stance.
So, at least one FA and one Admiral needed. An older Marine is most likely to get the FA job in my opinion- Fujitora, or maybe Kuzan returning especially with this SWORD revelation. Although he might be happier on his own now. Or get knocked off for being a mole by Blackbeard
Smoker probably will still end up Admiral in strength, but I do think he'll reject a promotion.
I can't see Helmeppo or Tashigi being strong enough.
Sentomaru and Drake are just maybes. It's feasible enough they could end up an Admiral, but I don't really see it.
Then there's the wildcard RA to consider. Once they overthrow the World Gov, would they want to join the Marines to make sure actual peace and justice was being kept? FA Dragon and Admiral Sabo? Don't think it's all that likely, but it's something to be considered. If I was forced to guess, I'd say
FA- Fujitora
Admiral- Ryokugyu, Coby, Drake(?)