#1 Charlotte Flair

Every other option is either unlikely or unwanted. That leaves us with the obvious choice of the Queen. Charlotte Flair is one of the greatest female wrestlers in history and has been the champion a countless amount of times.
She is getting on the WrestleMania card in a major match, but it remains to be seen just which one it would be. Becky Lynch will face Ronda Rousey for the Raw Women's Championship at Mania but Charlotte seems to have stuck her nose in that business, attacking Becky on Smackdown.
Sting in WWE once again? More details HERE
There is a good chance that she is added to that match to make it a triple threat, but that would leave a gap in the star power of the Smackdown title match. Asuka could defend her title against any of the previous names mentioned, but it would not feel nearly as big.
If Charlotte is not added to the Raw title match, she is absolutely the one who will face Asuka at WrestleMania. It would be a rematch of possibly the best match at last year's WrestleMania, with the added element of Asuka wanting revenge for the end of her streak.
This is the only possible WrestleMania match for the Smackdown Women's Championship that would feel like a big deal on a presumably stacked card.