#2 Shayna Baszler
This is another obvious one. With Ember Moon entering last year as champion, it seems likely that Baszler will enter this year as champion (Bianca BelAir will most likely lose at TakeOver: Phoenix the previous night). Aside from that, WWE insists on doing this four horsewomen feud (for reasons I still can't quite fathom) and the Royal Rumble would be an ideal venue to tease that eventuality.
#3 Io Shirai
Speaking of the NXT Women's Championship, the Royal Rumble would also be an ideal venue to begin the next feud for that title, which will likely feature Io Shirai, the Mae young Classic runner up.
Shirai's entrance would add a lot of excitement to the Ryoal Rumble match, as her high speed offense would pick up the pace during a match that can lag. It would also offer some interesting teases for future dream matches with the likes of Charlotte Flair or Ember Moon.