Here's how I would book this if I were running the show. Yes, fantasy booking, sue me.
This assumes that Brock does have some sort of long-term contract with WWE. However, he may fight for UFC in the interim during a gap in this plan, but it does require him to return down the road for at least a few dates.
At Wrestlemania, I would have some sort of momentous screw job that turns Reigns heel and causes Rollins to leave Wrestlemania with the title. You can have as many people involved with this as you think is necessary - J&J Security, Kane, Big Show, and so on. Whatever it takes to screw Brock out of the title and get it off of him for a while.
The next night on Raw, Heyman comes out and delivers a promo. If Brock is going to UFC, Heyman flat-out says so. If Brock is just going on hiatus for a few months, he gives some other reason for it.
You can have either Rollins or Reigns as your heel champion for a few months. Regardless of who it is - I'll say it's Rollins - you eventually have one of them face Bryan at Summerslam. Have Bryan win a title shot at some interim PPV or something.
At Summerslam, you have Rollins (c) vs. Bryan for the title. It starts off fairly clean, as you know the two of them can put on a pretty good match, but eventually the usual Rollins stooges start interfering. You have the ol' ref bump and then J&J and Kane and Big Show interfere for Rollins, pounding the shit out of Bryan.
Brock Lesnar runs in from the crowd, F5's everyone but Daniel Bryan, picks Bryan up with one hand, and puts him on top of Rollins. He exits the ring, 1-2-3, Bryan is champ.
Next January, when Brock is done with his UFC commitments, have him be a surprise entrant in the Rumble and win it. Meanwhile, Bryan is managing to hang onto the title versus the plethora of heels.
Main event, WM 32, face Bryan vs. face Lesnar. They can give interviews of mutual respect - Bryan respecting Lesnar as a pure physical force and Lesnar giving Bryan respect as being ridiculously tough for his size (but giving no doubt that Lesnar will still win).
That's how I'd book the thing. Aside from a single SummerSlam run-in, Brock gets 10 months off for a UFC comeback if he wants it and it all culminates with Lesnar vs. Bryan at WM 32.