It's entirely feasible that FOX have asked WWE to bring Brock Lesnar back to the company because he's seen as a legitimate fighter following his UFC career and that would give WWE some legitimacy when it's being promoted on their Network.
That being said, Brock Lesnar will never be a full-time Champion, he won't wrestle at smaller pay-per-views and he won't always be available for SmackDown shows because WWE can't afford how much he charges, which means that he would become a part-time Champion once again.
When Brock was Champion before, the mid-card title became the most prestigious title because he wasn't around to defend the Universal Championship at every pay-per-view, which isn't a position WWE wants to get itself into again.
SmackDown Live has become a place of opportunity under the control of Shane McMahon and this switch over to FOX shouldn't take this away. Lesnar is a huge pull for the Network, but he isn't enough of a pull to make him Champion once again and in turn, send a message to the WWE locker room when it comes to opportunities.