WWF/WWE has done cross promotions like this in the past to great success. And this time it would be with someone who can put on a match, even though he would be hella rusty. It's also doing a great job creating positive buzz for the wrestling business, something that has been hard to do after that dark shadow Chris Benoit cast over the industry, especially in the eyes of non wrestling fans.
It could happen, and sending The Undertaker to the fight to create buzz was a stroke of genius, it kind of made it feel like a WWE invasion. Will it happen? I don't think so, Dana White has stated in the past that he respects Vince McMahon but in every interview has where a crossover has been discussed, makes it clear that the UFC and WWE are two completely different monsters.
I love UFC and WWE but for different reasons and Brock being Undertakers Wrestlemania opponent while exciting in theory would seam too gimmicky to me as I'm sure it would to a lot of other fans. To me it makes more sense to have him face a WWE performer to do a really good job of sending him off and putting over an up and coming talent. And out of all the rumors, I'd like to see the Sheamus match, they've put a lot into this pale HHH clone, so they might a well go the distance, otherwise making him a two time champ seems a little insulting to us fans.
As for Brock, I can't wait to see his comeback fight. He strikes me as a fighter who learns from his mistakes and improves on them. I think when he fights again he'll be a more controlled and disciplined bat out of hell and really cement his place as an MMA great. And honestly a WWE one shot would cheapen all that he's worked hard to create in the UFC.