Initially, Brock Lesnar was expected to make his UFC return in early 2019. The plan was to have "The Beast" challenge Daniel Cormier for the heavyweight championship. While that can definitely still happen, Lesnar's new deal with WWE might put that in doubt.
Lesnar is still undergoing testing with the United States Anti-Doping Agency (USADA), a requirement for all UFC fighters, however, Lesnar must deliver six months worth of clean testing due to his suspension before being eligible to fight again. However, due to Roman Reigns' recent leukemia diagnosis, Lesnar once again won the Universal Championship at Crown Jewel.
With his new WWE deal, Dave Meltzer of MMA Fighting reports that he'll be in demand through April for WrestleMania 35. With that being said, it puts his heavyweight championship fight with Daniel Cormier in doubt for early 2019:
“Lesnar agreed secretly to the new deal, and won the pro wrestling championship again. The new deal is for multiple shows. But its length of time has been kept secretive. But it is known that Lesnar is scheduled as late as April of next year for WrestleMania.”
Cormier is ready to retire from MMA and would like for his final fight to be a money maker opposite the likes of Lesnar. Should Lesnar's WWE obligations prevent him from fighting before Cormier's 40th birthday in March - Cormier's targeted retirement date - "DC" would likely accept a third fight with Jon Jones.
Lesnar's deal with WWE isn't exclusive, however, and he is allowed to fight for the UFC while still under contract with WWE. It should also be noted Lesnar has yet to sign a bout agreement or contract with the UFC as of this writing. The next several months will be very interesting in regards to what Lesnar decides to do next.