Rey Mysterio's current contract with Lucha Underground is reportedly up after season three, and both WWE and GFW are actively pursuing him to join or in WWE's case return when he becomes a free agent, according to SI.com’s Justin Barrasso.
Mysterio was of course up until his departure on February 26, 2015, the WWE's biggest drawing name in the Mexican market, an area which WWE has been trying to revitalize since without much success. In addition, Barrasso notes that a return to WWE for Mysterio could benefit Dominic his Son who is seeking to break into the industry and with the company's development programs at the WWE Performance Center, it could be hugely beneficial to his career in the ring.
On the other hand, GFW’s lighter schedule might sway Mysterio who will be 43 in December and with all the latest drama with Alberto El Patron the company may seek to find a new star to appear to their Latino market.
As for Lucha Underground, a fourth season has yet to be announced so it's looking unlikely at this stage that Mysterio will stick around. If he does leave LU which will be after season three ends in September he will have a 90 day no compete clause which realistically means we probably wouldn't see Mysterio back on our screens until early 2018.