I've said it before, and I'll say it again. And the Undertaker marks usually will come out in full force, but oh well.
Cena becomes champion the very first night he comes back. The smarks will be livid. Undertaker becomes champion next PPV. The smarks will be besides themselves with joy. Why the difference? Got me. But, it's important.
Both men carry the title to the Royal Rumble. Both men win their Royal Rumble match, and then are surprise entrants in the Royal Rumble. Undertaker and Cena are last two in the ring. Cena throws out Undertaker. Which leads to the match at Wrestlemania 25, Cena vs. Undertaker. Undertaker's streak vs. Cena. Title vs. Title.
They have the match, a good 20-25 minute affair. Undertaker chokeslams Cena, Cena kicks out. Cena FUs Taker, Taker kicks out. Cena gets pissed and cheap shots Taker. Taker shrugs it off (no-selling isn't hard for Taker), and Tombstones Cena. Cena kicks out. Cena puts on the STFU. Undertaker taps.
Undertaker taps out to John Cena, at Wrestlemania...Wrestlemania 25. For the title unification. The smarks threaten to blade themselves.
The next night on Raw. Cena comes out. He cuts a short promo, going heel and talking about what a bastard the Undertaker is, and calling the fans worthless pieces of garbage for not believing in him, and not supporting him throughout the last few years, booing him during what should have been some of his most memorable moments. Then, he closes his promo with "You people booed me against the Undertaker. But I don't care. Because I'm the champ, and the CHAMP IS STILL HERE".
There, you have heel John Cena. The good wrestling fans will boo him because he's heel, and the smarks will be so pissed off about making the Undertaker tap out at Wrestlemania, it won't even be cool to cheer him.
Heel John Cena. It'll never happen though, but we're talking soda cups being thrown in the ring heel turn.