I can say that I wished for it from the moment I found out he was waiting for Joy Boy. But whether this will really happen depends on several things.
First of all Oda's plans for him. Did he just use him as a minor character whose role would end now that Zoro defeated him? Could be. But then again, there are so many little clues Oda has left, like that Lunarians used to live on the Red Line, that there is enough material for Oda to tell us a full story about that tribe if he wishes to, just like he did with Kuma.
Then, it also depends on the fact that Kaido is really dead. If by any chance, Kaido managed to survive that magma, and maybe even gained access to Pluton underneath Wano, then King will have a great dilema. He knows Joy Boy has beaten Kaido, since that's the rumor that was spread by Yamato. He could even suspect that this is Luffy, but he can't know for sure. On the other hand, King is so loyal to Kaido that if he is alive it would be near impossible for him to turn Kaido down. So if that's the case, I can't tell what his decision will be.
Even if Kaido is dead, it will be hard for King to join Luffy. King is a proud man, he was beaten by Luffy's subordinate (or so he will see it) and sustained incurable damage. Even though he admitted to respect Zoro, it will not be easy for him to cooperate and fight side by side with him. Luffy also killed the man who not only saved King's life, but kept him safe under him for decades. Even if he knew it in advance, as Kaido himself told him it would happen, it would be hard to simply shift allegiance to Luffy.
It's not going to be easy on King, but he believed in Joy Boy ever since childhood, and if this belief means to him that his people will be saved, or even simple vengeance against the world government for banishing his people and eliminating them, then he would make the effort and join the straw hats.