There could be some crossover promotional opportunities. It's tough because I think a lot of the "personalities" in MMA/UFC don't crossover that well to WWE. It's a different world between being a big personality in the octagon versus cutting wrestling promos.
Seeing Ronda Rousey's second run in the WWE pretty much bomb makes me wonder how most MMA superstars would fare. Conor McGregor would be able to cut a great promo, as long as he didn't resort to saying how "wrestling is fake" and insult people's intelligence. But guys like Israel, Jones, Cormier, etc. don't cut wrestling style promos. You need to be a Chael Sonnen or Conor McGregor to do that stuff. Even Nate Diaz would feel out of place.
Plus, a lot of the time, MMA guys like Junior Does Santos, Cain Velasquez, and Daniel Cormier, who we all know are huge badasses and could destroy 99.99% of the planet in one-on-one fights, don't look that impressive or imposing under the WWE lights. Conversely, there are few if any WWE wrestlers who wouldn't look completely amateurish and embarrassing in an actual MMA fight, and an injury there could put them out of action for a long time.
So I don't think we will see any Forbidden Door type events with WWE vs UFC, unless they do some celebrity stuff similar to the competitions they used to do on The Ultimate Fighter, like having them compete in different competitions or Family Feud or something.
But having UFC fighters try to cut WWE promos or even wrestle a real match, or having WWE wrestlers try to get in the cage and fight... neither would end well. Hearing them talk about retired MMA fighters in the WWE made me a little nervous.
