To be honest I have no idea what Oda wants to even do with Kid. Law usurped him in popularity as a rival supernova, he isn't as threatening an adversary as Blackbeard or Shanks, he doesn't have the wild card gag to him like Buggy, and he seems completely irrelevant against the backdrop of the World Government.
I feel like just making him eventually become another ally under the Strawhat grand fleet would be a humiliating and borderline disrespectful trajectory for his character. Him being killed by Oda's self-insert is almost a more respectful end for him than that. Because I don't trust Oda to make Kid seem like a genuine threat for the PK race.