Brock can't wrestle John Cena forever, Seth Rollins is a heel and the two of them won't wrestle unless it's a triple threat involving a third man (Cena), the Big Show is a terrible choice, Rusev is a heel and that match would make no sense, etc. Dean Ambrose is crazy over right now and is almost certainly not winning the Royal Rumble and headlining Wrestlemania anyway, so why not pit him against Brock Lesnar before then? With Bryan out, Punk gone and the Cena feud having been exhausted through, Lesnar needs some good opponents till Mania and Ambrose vs Brock would make for a compelling rivalry. Ambrose is supposed to be this fearless, take-on-any-challenges type of guy. It would fit his character to a T to be insane enough to call out Brock Lesnar and not back down from him. And then the match itself. The two have a hell of a brawl with one another and Ambrose takes a hell of a beating from Lesnar, but he keeps coming back for more. Imagine if he left a bruise on Brock Lesnar (remember Brock having a bruised eye the night after Wrestlemania XXX?) Also imagine he spends so much time in the Kimura Lock (which he refuses to tap out to, btw) that officials are concerned after the match that his arm might be broken and yet he refuses their help. Hell, maybe his arm IS broken and yet he still rejects their medical assistance, insisting on walking out on his own volition. As a one month thing (the Rumble probably being the best place for it), I think this could make for a great encounter that would get Ambrose even more over.