#1 Asuka

This may not seem highly probable at the moment but will definitely the shock the WWE Universe if it does happen at the upcoming event. This is one of the safest ways in which WWE can make Becky Lynch lose her title match to Asuka. Becky Lynch will then be able to enter the Royal Rumble match and hopefully win it.
As there are two more PPVs in between Wrestlemania 35 and Royal Rumble, Becky Lynch can then continue her feud with a heel Asuka for at least one more PPV (say, Elimination Chamber) before moving onto the flagship show Raw to face Ronda Rousey at Wrestlemania 35.
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A change in character will be ideal for both Asuka and the Smackdown Live Women's Division. It is heavily being speculated that one or both of Smackdown's top stars - Becky Lynch and Charlotte Flair - are set to jump ship to Raw in the near future, which leaves Smackdown Live without a proper main event level heel.
If the triple threat between Becky Lynch, Charlotte Flair, and Ronda Rousey is the plan for Wrestlemania 35 then turning Asuka heel now would be the right decision going forward.
Heel champion Asuka can then battle the likes of Ember Moon, Naomi, or Carmella at Wrestlemania 35.