Maybe Lesnar said something to the effect of "I'll think about it if I lose to Cain" but that's just me speculating.
This would be a huge move for WWE but I think there would be some conflict on the result of the match. For Brock to come back and lose would hurt his image, whether wresting is fake or not..it would change how people look at Brock. And UT damn sure ain't giving up his streak to Brock, let alone WWE make a now MMA fighter go over their biggest remaining legend.
I don't know the details but I do know that alot of people were pissed at Brock for leaving how he did, after WWE did so much to build him and having him beat guys like Rock, Hogan, and I'm sure Undertaker as well. It's been so long that i don't remember if he beat UT, but as a locker room leader he probably ain't too high on Brock bouncing either.
I hope it does happen.