I don't think Law will join, but I don't think it would be that much of a stretch if Oda decided to have him join. Luffy seems to consider their alliance as Law being part of his crew, and has called him nakama already. He isn't interested in becoming Pirate King since he ultimately only cared about killing Doflamingo with the understanding that he would likely die in the process. Instead, he got a new dream inspired by Corazon to understand the Will of D., the answer to which is on Laugh Tale and requires Robin which I think is the biggest argument in Law's favor of joining.