#3 Humberto Carrillo joins SmackDown as Natalya joins RAW

While this would break up her current tag team on SmackDown, Natalya is better used with younger talent at this point in her career. Rather than benefitting from the inevitable dissolution between Shayna Baszler and Nia Jax, Nattie needs to be used to help newer/lesser talented stars.
Eva Marie is apparently returning to WWE on RAW, and unless she turned into another version of Asuka during her time away from the company, she'd need to be paired with someone. Charlotte Flair's character is all about herself, while Asuka has done enough of playing second fiddle.
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The women's tag team division in WWE hasn't been booked the best since Alexa Bliss and Nikki Cross dropped the titles. The Riott Squad should have won the title at the Show of Shows as they were booked as the underdog team. Instead, it looks like WWE wants the two veterans to be the team to dethrone Jax and Baszler.
Nattie would be better served as a confidante/teacher for the returning Eva Marie. The Queen of Harts might not have had the illustrious career of one of the Four Horsewomen of NXT, but she is a respected veteran. At this point, her helping others along is the best use of her talents.
Sending Humberto Carrillo to SmackDown seems like the best move for him. He's exciting in the ring but has trouble standing out as a character. On SmackDown, he can just wrestle rather than trying to develop a loaded character. If they wanted to, WWE could also have him ally with the Mysterios since he protected them when Seth Rollins terrorized the family.
Big E isn't likely to take his title back from Apollo Crews so SmackDown will need some new faces. There could be some additions to the blue brand from NXT as new faces showed up on both RAW (Mansoor, Marie) and NXT (Asher Hale, Swerve's group) this week.
Carrillo could also enter into a tag team with someone or he could fight on Mysterio's behalf if and when Dominik turns on Rey. SmackDown seems to rely more on in-ring talent rather than grandiose promos and angles for some of its stars. That is perfect for the talented Luchador.