Akainu isn't evil, just willing to go to nearly any extreme to achieve his vision of justice. He's also the strongest character alive.
Pirates as a whole are boring as antagonists, most of the reason I read OP is for the subplots surrounding the CD's, WG and marines, and I hope they become the main villains front and center, rather than in the background.
Jinbe is a great character and would be a good addition to the strawhats with his mix of strength and competence.