I'm going to make like @Messiah and make a wild WWE prediction for 2015.
Most people seem to think that Reigns is winning the Rumble and main eventing Wrestlemania, even though he's likely not ready for this kind of push yet. He needs more character development, and he needs to work on his promos. He doesn't seem like the right person to face Brock.
I agree that it's too soon for Reigns, but that doesn't mean WWE won't go ahead with his push. When they get an idea in their heads, it's hard to talk them out of it. But I'm going to make the wild guess that he's not going to be facing Lesnar. I reckon he could be facing Rollins for the title. It's a more personal match for him, and Rollins is more over as a heel than Lesnar is right now so it'd probably get Reigns a bigger reaction.
I'm guessing that Cena wins at the Royal Rumble to capture the championship. Then Rollins cashes in Money in the Bank afterwards and wins the title from him. Meaning Rollins would be champion heading into Wrestlemania. Reigns wins the Rumble to set up the main event.
Lesnar either leaves the WWE for good (a lot of people seem to be expecting this to happen soon), or he sticks around for a match against Bryan at Mania without the title on the line.
I could be way off, but it strikes me as possible. If they really want to shake things up, this wouldn't be a bad way to do it. Reigns would be more likely to get over as champion if he was facing Rollins. Provided he's not the guy to eliminate Bryan from the Rumble he might actually stay in the audience's good graces.