#4 There are new feuds on the main roster and some holdovers from NXT
In terms of the meat and bones of the NXT women's roster, Shirai has faced most of them. She's battled Dakota Kai, Raquel Gonzalez, Toni Storm, Shotzi Blackheart, Zoey Stark and many others. Shirai has also faced Charlotte Flair, Sasha Banks, Rhea Ripley and Bianca Belair from the main roster.
If Shirai does join the main roster after WrestleMania 37, it should be on SmackDown. RAW just added Ripley to Asuka, Shayna Baszler, Flair, Nikki Cross, Alexa Bliss, Nia Jax and Peyton Royce. They already have a lot of depth and adding Shirai would lessen the chances of the middle of the roster advancing.
Joining SmackDown would add another main-event level woman to the division. Sure, she's already faced Banks and Belair, but there are other feuds to be had.
She'd likely enter into a gatekeeper type of angle with Natalya since Nattie is often like the Dolph Ziggler or Cesaro of the women's division. Newer stars from NXT get their feet wet on the main roster against trusted veterans like the Queen of Harts.
The SmackDown women's roster also features the likes of Carmella, who loves herself, Bayley and members of the Riott Squad. Tamina and Billie Kay are also part of the division, but Bayley did well in welcoming Belair to the main roster. Unfortunately, the former SmackDown Women's Champ doesn't even have a match at WrestleMania 37.
The division on the blue brand is a lot different than that of RAW or NXT. RAW and NXT have good depth throughout their rosters. SmackDown, however, has four main-event level women - Banks, Bayley, Belair and Carmella. Mella lost several title matches to Banks but is still a good character.
Bayley had a long and fruitful reign but was defeated by Banks. Whichever woman walks out of WrestleMania 37 will need new challengers. While challenging for the title shouldn't happen right away, Shirai would be a great addition to Friday nights.
If she went to RAW, she could rekindle feuds with Ripley, Flair and Baszler. Shirai could also face Royce, Cross or Naomi. There are just more opportunities to test herself against the best on either RAW or SmackDown than there are staying on NXT.