If cena is wwe champion going into wrestlemania, what if?

Not necessarily, if a RAW superstar was to win the Royal Rumble, they could always have them face the World Heavyweight Champion. I don't think they will make it a triple threat, it would ruin the match, think about it: The Rock vs. John Cena VS. Dolph Ziggler or The Rock vs. John Cena vs. Alex Riley, it would just ruin the prestige of the match. So to answer your question, most likely they would have a RAW Royal Rumble winner face the WHC. IE: World Heavyweight Champion Christian vs. Dolph Ziggler