B.B is definitely one, though I feel he will most likely kill Shanks/Sabo in this arc before that conflict, if there was a pirate Luffy would fight soon it would be Eustass.
I think Luffy will defeat an Admiral before his BlackBeard confrontation.
Antagonists include:
Im, Celestial Dragons, Akainu, and a popular theory among fans; Rocks D. Xebec - though I feel he will either return as a Cyborg via Vegapunk or will feature in a post manga canonical film.
Next arc is gonna be chaos, since there’s so much exposition, history, character development & world building to accomplish.