Personally i see no good candidates for next generation of Admirals, unlike SNs that got good chances to be next top pirates, Oda didn't the same with Marines.
Ofcrs there is Koby with good claim, bt he is more like Plot guy, will somehow become one offscreen. Maybe vacant seats will play a role or some very OP df.
Drake recently got revealed to be still working for the Marines, and is legit candidates for next generation. Being undercover agent n Captain of swords that is working against WG also helps his claims, that he is gonna play big part in it.
Smoker, i may be underestimating him, bt Oda did him very bad in PH. His df also isn't at same level like Admirals and also can't see him developing Advance Haki at this stage, when he might not even get Extreme fights or training. Mayhe Vacant seats will also again play a role here.
Fujutora can remain an admiral and is also a prim3 candidate for FA.
And Another being Kuzan, who originally was supposed to be the FA.