One thing from the last decade, Vince McMahon loved working against his top talents. He locked up with the likes of Hulk Hogan, Shawn Michaels, Triple H and many other WWE wrestlers. Vince even had a run as ECW champion! Given this self serving booking, you have to guess that McMahon would've had booked himself against Lesnar at some point. Brock would have represented his biggest star and from a machismo point of view it would've been a kick for Vince. You suspect he would have liked to lock up with Brock on the big stage like Mania. For Brock, in the long run he definitely had that type of run in him. He'd have been like a badass Stone Cold character had he stayed in WWE and after working alongside Mr McMahon in storylines it would've made sense for him to then eventually butt heads with the boss. Unfortunately, Vince is way too old for a physical guy like Brock nowadays.
