According to a report from The Wrestling Observer Newsletter (via PWPix.net), when WWE originally announced a Triple Threat Match for the Universal Championship at Crown Jewel between Roman Reigns, Brock Lesnar and Braun Strowman, their plan at the time was for Roman to retain the title and for Brock Lesnar to be written off of WWE programming for a while.
However, due to Roman's leukemia diagnosis, the decision was made to put the championship back on Lesnar.
As for Lesnar's future with WWE, he is obligated to perform in at least two more matches for the company. He is currently scheduled to face WWE Champion A.J. Styles at Survivor Series in a rematch from last year, and then he is scheduled for at least one more match after that.
His new deal with WWE will also allow him to return to the Ultimate Fighting Championship, as Lesnar is heavily favored to be the next opponent for current UFC Heavyweight Champion (and current Light Heavyweight Champion) Daniel Cormier.