According to Wrestling Inc. (via Wrestling Observer Live), a deal between Mysterio and WE has yet to be signed but current expectation have him returning in September in the wake of SummerSlam.
After a lucrative cameo at the Royal Rumble fan immediately began to clamor for Mysterio to join WWE's roster. Rumors indicated that Mysterio didn't want to work WWE notoriously rigorous schedule and that hang up delayed any type of agreement. However, Mysterio still found himself attached to WrestleMania rumors. But before they could reach their fever pitch, Mysterio tore his bicep and hit the disabled list.
The future WWE Hall of Famer is healthy now and is actually set to compete at an NJPW event June 9. Mysterio apparently has a busy summer scheduled and that's why his WWE return will have to wait for until the early fall.
In an interview with the Mirror in March, Mysterio heavily foreshadowed a return to Vince McMahon's company.
“I want to say that there is a big possibility that I will be back. [We’re] figuring out what the situation is, you know from both ends. But there is definitely a positive feedback on behalf of the fans, on behalf of the WWE staff and behalf of myself man. I think we’re possibly going to sit down and negotiate something where we can both work together and have a good time and enjoy," he said.
There are a number of interesting spots to stick Mysterio upon his return. 205 Live seems logical—after all, Mysterio is a cruiserweight demigod—but WWE may have bigger plans. Mysterio himself may also have higher ambitions as he recently hinted at created a significant Latino wrestling faction.
No matter what, WWE fans will be happy to lay eyes n Mysterio. While he may not have the 16 World Champions of John Cena, Mysterio owns one of the more unique relationships with WWE's live audience. He mentioned as much after his loud guest appearance at the 2018 Royal Rumble.
It’s incredible. It’s been almost three years since I’ve heard that WWE fan reaction and to be honest with you, a lot of people say it’s luck and a lot of people say it’s awesome to hear that but I’ll tell you, I’m truly blessed. I’m truly blessed to come back and realize the fans don’t forget about you, and not only the fans but the WWE family. This is a universal reaction and it feels good, it feels great," he said.