My big problem with theories like Law or Bartolomeo joining is that it's not just about how that character may feel or even interact with Luffy, it's about him becoming a strawhat and actually joining the crew as a member that bothers me. Bartolomeo may adore the Straw Hats and if asked to become one would most probably say yes on the spot, but I don't ever, ever see Luffy realistically asking Barto to join the crew. Why would he? Plenty of people have helped the Straw Hats before and not been asked, and I just don't see Barto ever being invited. Furthermore, I know he adores the Straw Hats, but I don't see the point of a character to come on the goddamn ship and be part of the main cast and eat up screen time when all he does is gush over all of them. He would never feel to me like a nakama who if he got captured they'd all risk everything to get him back like Robin.
Same can be said for Robin when she instantly joined the crew I guess. Of course there will be character progress so I'm not worrying about the gush part. Btw, I like Barto to join but I also don't think he will.