My point is not that you specifically, but people as a whole go way too easily towards the 'next Strawhat' thing with every character that's promising. How many new women have appeared in this arc alone? How many of them had interesting abilities? How many of them had different personalities from Nami and Robin? Her backstory is practically non-existant as of now, so nothing much can be said about that either.
Fact is your reasons are pretty much two times nothing and thus there aren't any real arguments as why now Sugar would have more chance of becoming a Strawhat than someone like Violet. It's possible, but that's just because there isn't anything that states it wouldn't be possible. I just find it slightly annoying that many people go so easily to the 'this one is going to be the next Strawhat' thing while there isn't anything substantial that makes those characters different from other.
You can of course wish for it, but that's slightly different than saying you think she is going to join just based, like usual, on nothing substantial. I think it makes the entire discussion of 'who is going to be the next Strawhat?' redundant as in the end it pretty much comes down to guessing and wishful thinking.
It's obvious that we're going to have different opinions about this. I personally think Sugar stands out from the other females that have been introduced in this arc, and you, well, you probably don't agree with me. Her ability to turn people into toys is one of the most fascinating abilities so far in the series, or so I think, and it's far more interesting than the ability to turn oneself into any weapon or the ability to read minds.
The 'problem' I have with the other females is that they're too open to romance, with the exception of Rebecca. Viola is too friendly with Sanji, and there are times one may wonder if she has feelings for him or not. This alone sends Viola down under most candidates on my list of 'next Strawhat'. I'm not saying it's impossible that she'll join the crew in the end, but because she's the princess of Dressrosa, I find it more likely that she'll end up staying.
The problem with Baby 5 goes without saying. She would obviously go as far as to marry someone if she's ever under the impression that she's needed. It would get tiring if she'd get emotional every single time a person from the crew asked her for help or ordered her around. And I can't imagine how she would react to Sanji, who'd probably flirt with her every single day. To me, it's these small things that decide whether one become a crew mate or not.
I don't really have anything against Rebecca as a character, but still, there are things about her that bothers me. She holds back while fighting; she's a pacifist, and she doesn't believe in fighting. She can't hold back when she travels around with the Strawhats. Another thing is that she looks exactly like Nami, except her coloring. I don't know about you, but I want each crew member to have unique features and looks. So, no, I just don't see her joining the crew.
So, out of all the females, Sugar is the only one that stands out, or that's what I think at least. It's clearly just my wishful thinking, but that doesn't mean I don't think it will happen. You said that we can't say much about her backstory, but we can actually. Her devil fruit abilities made her age freeze, so who knows how old she really is. For all we know, she could've been born centuries ago. If that is so, we can only imagine how lonely she is if the people around her died one after another. That's why, once again, I think her backstory has lots of potential.