*Disclaimer my alternate theories have 0 basis and are only off the top of my head.
@Darkstorm:
Why would he have died unless this is supposed to be some alternate "SH never existed" thing or are you talking of old age.
I don't know - pure speculation but if Kuro's plan went through I suspect that Usopp would have become involved and killed because of it.
No, he joined to find All Blue.
That he did - But Zeff also implied that he would not be a better Chef till he travelled the world or something similar - and as such he is achieving that now.
No, he joined(you seem to be going from choppers perspective here) because of the romance Hiriluk had instilled in him with the idea of pirates, and because luffy didn't care that he was, as he saw, a "monster".
To be honest in the unedited post I skipped mentioning Chopper forgetting all about him - I know we're not suppose to base the characters joining only on their skill sets - but I was trying to boil down their uniqueness to a blunt level - unfortunately doing so also lost a lot of actual truths to their joining sorry for messing up this one.
I'd say more like interested in him, plus she had nothing else to do. She had no other clues to the rio poneglyph, and shortly before had pretty much said she didn't really have a reason to live anymore.
I was also going to go with he was the strongest person she had found to protect her - but I just couldn't really justify that position. If Robin was going to live she needed to live somewhere she wouldn't be hunted which meant with someone powerful - but it kinda sells her short thinking like that
Regardless in make believe land - had Crocodile not been defeated after she knew the Ponyglyph held no information for her she probably would have continued to search if Croc was going to continue in Alabasta.
My whole point of my post was - comparing who is/should/could join on previous SH's joining arcs is futile - each will have their own unique reasons for joining and ways of joining. Of that, I know most people here will agree …. barring those who have more simpler tastes.