According to a report from The Wrestling Observer Newsletter, the word going around backstage is that current WWE Universal Champion Brock Lesnar actually does want to stay with WWE, and it's basically a similar situation that happened a few years ago when Lesnar's contract was about to expire.
However, in the event that Lesnar does not reach a new deal with WWE, their plan right now is to proceed under the assumption that he will be gone after WrestleMania 34. Look for WWE to really try to rally the fans behind Roman Reigns in the coming weeks. Although there's a decent chance that Lesnar will end up re-signing, WWE officials would like to create the perception among the fans that his departure would actually be a good thing instead of having people think that WWE's loss is a huge gain for UFC, which is likely where Lesnar would end up if he does leave WWE.
As of right now, it is still expected that Reigns will be going over Lesnar at WrestleMania 34 for the Universal Championship.