It really to me needs to be Reigns over Lesnar at Mania. You have to create that amazing moment to make Reigns. You can then probably make Rollins by having him come out, cash in, take advantage and "ruin" Reigns's "WM moment" and off to the races you go.
I think Reigns is pretty good, he isn't amazing in the ring, but he's clearly a case so far to me of what the smarks like doesn't matter because the average fan likes him and his stuff and seems pumped every time he gets into a good position for the belt or similar. It's possible a lot of that is the "we'll pull for anyone you like who isn't Cena, please Vince!" and he'll flame out once he gets the belt, but we really won't know until they try. The future is now, it HAS to be. Cena is hurting, he's not getting younger, and god forbid that turn into a serious injury that ends his career before he can pass the torch to anybody.