#2 Have Rousey lose a match
Believe it or not, there is no better way to kill a superstars momentum, even just for a moment, by handing them the first loss of their career. Just take Asuka for example, who suffered her first loss to Charlotte Flair at WrestleMania 34 and hasn't been much of threat since then.
With that being said, Rousey losing a match to Nikki Bella before the Evolution pay per view would go a long way in helping fans feel like the title is really up for grabs. Not only will that help WWE justify making this match the main event of the pay per view, it also will be a huge swerve and excellent storytelling.
Beyond that, a loss for Rousey is a huge crack in her seemingly unbreakable Armour and could be the start of a significant shift in how the company books her in the future. Of course, that's just going to lead to Rousey eventually losing to someone else, but it's also an effective storytelling device that will help sell this pay per view!