Under the assumption that the respective women's champions retain from now into Mania?
I'd call up Bianca Belair, have her win it and challenge Becky Lynch. I've always been of the opinion that in general, things like the Royal Rumble and King of the Ring should be used as star-making opportunities for people either on the cusp but need an extra nudge, someone who you have big plans for, or someone that could stand to receive a career rejuvenation/spent enough time outside of the main event scene so that it feels new when he or she returns. That last one is why I think Seth Rollins should win the Rumble on the men's side.
Not to reiterate that this person is a big deal ala Charlotte, when she's never NOT been the biggest deal in the women's division.
Because of that, I'm looking at people like Ruby Riott, Sonya Deville, etc. in terms of who I would have win the thing. People like Sasha, Bayley, Naomi, etc … they don't need a Rumble win to be legit. Just have them whoop ass for weeks straight like Becky did before she flipped the switch.
But Mania is not about those fully invested in the product. It's about casuals, what gets people talking. So obviously, the move is for Becky Lynch to win the Rumble and challenge Ronda Rousey.