King could be his opponent sure. Though I think Sanji's potential main opponent is something still quite ambigous at this point, as I think Oda is purposefully keeping it that way. Time will tell I guess.
I agree with you about Sanji's relationship with Kinemon, and what he has meant to the Scabbards. I mean he was the SH that decided to help Kinemon back on PH even though the other SH told Sanji to leave him. Sanji replied "I'll take responsibility". He sure took responsibility, bringing Kinemon with him, saving his life from the shark, saving the Mink race from extinction thereby allowing for the revival of the Scabbards and the resistance against Kaido/Orochi, saving Momo from execution and thereby preserving the hope and future of Wano, and now potentially heading to once again protect what Kinemon/Scabbards stand for.