The only "new Strawhat" I would be happy about is Vivi or any pre-existing character that has a long-standing relationship with the crew. Vivi had a special connection with the Strawhats, and I think there are next to no other characters in the story that share that connection. A big point of that is both the fact that she was with them as soon as they entered the Grand Line, and even more importantly, she was introduced when the cast of characters was fewer in number. This not only gave us more time to appreciate her, but also made her contributions (both interpersonal and plot-wise) feel more relevant and important. It is no wonder that love for Vivi has endured throughout the community when taking this into account.
Now, I am aware a lot of people think Yamato is cool, but the depth of her relationship with the crew and Luffy is simply incomparable to Vivi. Yamato's appeal is her strength, and though that will be relevant, it doesn't need to be as a crew member. Yamato's strength and loyalty towards Luffy can still be demonstrated as an ally who rolls up and fights for the Strawhats in the final war.
One could argue Vivi is the same, but I think with the introduction of the Lily subplot an increased narrative focus has been placed on her.
Do I think Vivi WILL rejoin? Nowadays I don't, but her joining would satisfy me more than any other Island's Princess or FotM.
To finish this however, I mentioned characters with long-standing relationships with the crew, so I think it would be fair for me to name one other. Smoker. Yes, he's a marine. No, I don't think he'd join. But he's been around long enough for me to feel like he isn't just some random addition for the final arcs of the story, unlike say Loki, who we have only just met.
These are just desires however. It isn't like Oda cares for any of that, and if he wants Loki, Carrot, Yamato, etc to join, then it'll happen.