Dave Meltzer is reporting that Lesnar may face Shane McMahon at WrestleMania.
This seems to explain why they had Lesnar hit Shane with the F5 at the end of SummerSlam. Another big part of the reason why they are possibly booking Shane McMahon vs. Brock likely has to do with McMahon being a big ticket mover during the weeks leading up to the match with The Undertaker for WrestleMania 32.
Ticket sales had picked up when he returned and when it was announced that he would face Taker. The Surivor Series match coming up with Lesnar and Goldberg is also still a "one and done" type of deal. There's no plans for Goldberg for Mania.