@King:
You don't have many other potential antagonists besides Admirals and Emperors left either. You got the 7th Warlord… and I guess that's it.
Potentially the list of possible villains can be extended beyond that if Oda wants to - certain Tenryuubito could have been pissed off at Luffy's actions two years ago and having lost faith in the Marines ability to carry out their "justice", they may have made the choice to take matters into their own hands and consumed some really powerful Devil Fruits or created an army of their own. I have always wondered why the Tenryuubito who have the ability to attain insanely powerful Devil Fruits don't consume any.
I am not sure if the CP0 will feature majorly again but I suppose there is a possibility that they could serve as major antagonist at some point.
The Impel Down level 6 escapees, unless Oda only intends to leave them to be covered by the anime production team in fillers/specials/movies, we may see more of them in future arcs.
Other notable pirates who wield Haoshoku Haki, Chinjao did say that the New World is filled with them, so potentially we may see a few new antagonist wielding Haoshoku Haki challenging the Straw Hat Pirates.
There is also the Gorosei, but I wonder how they will be dealt with at the end of One Piece - how powerful are they anyway?