Kid has enough profile to become a cyborg, it fits the character a lot. And who would be perfect for that? Queen!!! Did you ever think about Kid recruiting Queen? Queen could upgrade Kid, give him technology to store his metals in capsules, and endless possibilities.
Ulti and Page One are really cool and I'd really like to see them in the story. And they would have screen time more in Heart Pirates, but it's difficult because Law has no interest in acquiring power or being the strongest, doesn't seem to want to be a Yonko.