True, Taker (and HHH for that matter) is the last of a dying breed. I could see how people can say a legend should face Taker but I don't see it. Most of them can't go, have left wrestling, or would come back with no heat most likely. Cena is the top man of the company, whether people like it or not and we haven't had a Cena/Taker feud in YEARS dating back to '03 on Smackdown and even then, Cena wasn't the same guy then as he is now. Gimmick-wise, same with Taker.
Look, Cena's career (as of now) at this rate is reaching legend status that we will look back into 20-30 years from now and we will comfortably put him next to a Rock, Austin, Hogan, Flair, HBK, HHH, Taker, etc. Cena's only real challenge left at this point is to challenge the Phenom and his streak. He's the only guy left who can convince a crowd that he can defeat him at Wrestlemania. It will be even worse if he turns heel because then a possible win for him is imminent.