To RAW: Need of a big heel on the red brand
With the Draft over, it is clear that the red brand is lacking a big heel. Sure, there is Charlotte Flair, but it will not be wise to insert her into the title picture of RAW, given that she has already had a lot of matches with Lynch over this past year.
On the other hand, SmackDown has Sasha Banks, Bayley, and Lacey Evans, who perhaps are the most despised heels in WWE right now. Though Rousey fared well as a babyface, it was her badass heel persona, which was embraced by the WWE Universe. Thus, making her return as a face won't be the best decision.
If Rousey is to return as a heel, it should be on RAW, given the fact that the brand lacks a top female heel. With Brock Lesnar as WWE Champion and Cain Velasquez having already signed with the WWE, I guess the FOX executives won't mind Rousey going to RAW, or will they?
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To SmackDown: Already developed gimmick
Rousey's character development was perhaps the best in the women's division, until Becky Lynch's organic rise to the top as The Man. She started as the anti-authority babyface, who was hailed by many as the second coming of the legendary Stone Cold Steve Austin due to her involvement with Stephanie McMahon on more than one occasion. Towards the end of her first run with the company, Rousey turned into a dominant heel, which was something the fans had been clamoring to see. Heel or face, Rousey's gimmick has been over with the fans and doesn't need much of a development.
As seen in 2019's Draft, many up and coming Superstars have been drafted to RAW. The main reason behind this was to give these superstars more screen time, which will help them establish themselves and develop their gimmick. With Paul Heyman known to build main eventers out of new wrestlers, expect many more main eventers to come up soon.
But, Rousey is already over with the fans and is also a well-established main-eventer. So, it will be better for her to be on SmackDown, while other talents build themselves up on RAW.