2018 - Asuka won the first ever women's Royal Rumble. 2019 - Becky Lynch won the women's Royal Rumble. 2020 - the third female winner of the Royal Rumble was Charlotte Flair.
So who wins 2021? Let's look at the options.
If you're looking at the past three years, then one of the other two 4 Horsewomen (Bayley or Sasha Banks) have the best chances of doing so. Given that Sasha is the Smackdown Women's Champion, she won't actually be competing in the Royal Rumble. So, Bayley may have the best odds of winning. The only problem is that Bayley vs. Sasha just happened. It just wrapped up. Sure, we could revisit it, but WWE would need to make it fresh after the feud this past Fall. I don't think she will win, but I think Bayley will be one of the final four women in the match.
WWE allowed Steve Austin to win multiple Royal Rumble matches, so they could do the same with Charlotte Flair this go around, but I doubt it (and I hope not!). She won a Women's Tag Team title on her first night back in December. She doesn't need another Royal Rumble victory.
She's loyal and a good worker, but she's not at the level of winning a Royal Rumble and being the centerpiece of a feud going into Wrestlemania.
She's young and athletic, but she is also not at the level of winning a Royal Rumble and being the centerpiece of a feud going into Wrestlemania. I could see WWE pulling the trigger on her, though. Sasha vs. Bianca as faces? I don't know that this would work all the way through Wrestlemania. I think Bianca would have to turn heel, which is fine by me because I think she's better that way.
WWE could give this fan favorite her chance at a big moment. She's certainly won titles and battle royals in the past, but she's never had as big a win as the Royal Rumble would be. Naomi vs. Sasha isn't a bad idea for a Wrestlemania match. It's more of a face vs. face unless one of them turns heel, but that may be a nice change. Maybe some heel turn teases with neither one actually going full heel?
I've seen her name thrown around as a possible winner and I wouldn't even be disappointed with it. She doesn't have a lot of big match experience but Sasha could walk her through a match. Sonya came into her own in 2020 and developed a strong heel persona. She improved as a character and on the microphone thanks to her feud with Mandy Rose. I don't know that she'd walk out of Wrestlemania with a title win, but she'd be a solid heel for Sasha to feud with.
She got a Wrestlemania moment with Charlotte in 2020, but Rhea could be the one winning this year and challenging Asuka or Charlotte (assuming Charlotte wins the Raw Women's Title before Wrestlemania) or both of them in a triple threat. I just don't have an interest in anything involving Charlotte, but I don't have faith WWE will put effort into anything not involving Charlotte. So, I guess I'd settle for a triple threat.
She has a certain credibility to her, due to her size. Even if she's not the best worker, she could convincingly win a Royal Rumble on any night. I don't know how she lines up in any feuds, though. Asuka vs. Jax has happened several times before. It wouldn't be a bad match to have. If Nia won and challenged Asuka for the Raw Women's Title, then you could have a secondary feud between Charlotte and Shayna Baszler going into Wrestlemania as well. It'd be refreshing to not have Charlotte in the title picture (which means WWE will probably throw her right into the title picture).
Just like her tag team partner, Shayna could win the Rumble. She could face Asuka at Wrestlemania. She could face Flair at Wrestlemania (should Flair win the Raw Women's Title before then). However, I think WWE will simply have Shayna and Nia turn on each other in the Rumble match with a big 'payoff match' happening on a random Raw instead of culminating at a pay-per-view.
A returning Ronda Rousey would be my pick to win. She shows up, cleans house and goes onto Wrestlemania. Asuka would probably lose her Raw Women's Title to Charlotte Flair at February's pay-per-view and then it'd be Charlotte vs. Ronda Rousey (the match WWE wanted back in 2019). I'd actually prefer Ronda Rousey vs. Asuka, but I doubt WWE would leave Charlotte out of a match like that, so, if I want Asuka / Rousey, then a triple threat Asuka vs. Rousey vs. Flair is probably the only way it'll happen.