Alright, this is it. For all you people who want Cena to turn heel, this could very well do it.
Think about it.
Let's say Cena has been trying and failing for three months to recapture championship gold. Now it's the Royal Rumble, Cena's chance to go to WrestleMania, but he winds up being eliminated. By who? Yep, Undertaker.
After the elimination, he runs back into the ring and eliminates Undertaker out of frustration. The following night on RAW, Cena gets a title match, only to have Undertaker interrupt.
Then on Smackdown, Cena brings a coffin to the ring and dares Undertaker to come out and fight him. A brawl carries out and Cena comes out on top.
Then he stuffs Undertaker into the coffin and burns it. At the Elimination Chamber Cena and Orton are the last two left. The familiar gong is heard. The room is pitch black, Cena is laid out and Orton goes for the win.
Undertaker throws out the challenge, Cena accepts and their Road to WrestleMania will be filled with brawls, heel turns, and surprises.