I think Undertaker is still obviously popular, but if his physical state is that bad he can only wrestle once a year, it's time to do a retirement match and leave. And NO, the Undertaker's last match ever should not be against freaking John Cena. It'd be embarrassing for Taker and another undeserved Cena achievement. He's on his way to becoming the most accomplished wrestler of all time that no one liked. I don't know who I'd put him against but Cena definitely does not need putting over with a Taker win.
I feel a bit sad for Taker, if you see his interview during that UFC match where he confronted Lesnar, seems like he would have really liked to participate in those real brawls, but it came far too late for him.