I don't see a snowball's chance in hell of them letting Ambrose be the one to defeat Brock. Besides, they've got some creative issues to work out with his character (like namely the fact that they think being edgy these days means squirting ketchup in someone'e eyes or beating up a mannequin of your enemy) before pushing him to the top.
Well, they should let Ambrose be the one to defeat Lesnar. I don't care if he seems 'believable' enough because of his size/credibility or whatever. They just have to pull their heads outta their asses, prolly forcefully, and push Ambrose or Bryan (but doubtful he'll be back by January).