Let's go over this once more. What I'm saying is that there's a hella lot of space for Sugar's character to develop, while in Viola's case, her character is starting to run dry. We know of her past, which, in my opinion, pales compared to the average "sad" past. As if that wasn't enough, there's nothing that suggests she has some sort of dream, which has, up till now, been revealed during a member's flashback. Unless Oda decides to pull something out of his ass and give her some random dream, I doubt her joining will ever happen.
However, we know nothing about Sugar, and it's clear as day that she has a lot more potential than Viola. However, the reason why so many don't even consider Sugar, is because there's been introduced a lot of likable characters lately. Obviously, this is by no means a bad thing, but they outshine Sugar. In the Alabasta arc, there were literally only two candidates of Strawhats-to-be, and those were Vivi and Robin. If characters similar to, I don't know, Bartelomeo or Kyros were in the Alabasta arc, a lot of readers would quickly turn their backs on Robin.
If he'll ever join a pirate crew, it'd be the Heart Pirates. That's what I think, at least.