HHH showing up and preventing Sheamus from winning the Rumble would be the more likely scenario.
Evan Bourne and Christian are possibilities, though I don't remember exactly how long they said either would be out for. CM Punk hasn't really gone anywhere, he's simply resting a tweaked hip, his commentary is pure gold so I honestly wouldn't be too shocked to see him doing that for longer than you'd think.
Undertaker's return on the other hand will probably pertain more to Nexus or Kane than a Rumble resurfacing.
The fact that you'd even compare Christian to John Cena in the context of returning and winning a Royal Rumble is complete insanity.
No, I haven't heard anything about Christian... because he's Christian. He was put on the shelf by Alberto Del Rio, and that's most likely how he'll return... to perhaps if he's lucky take whatever mid card strap Del Rio is holding off him at the time. The fact that you can't see the distinction between someone like Cena returning to win a Royal Rumble and a mid card wrestler like Christian returning to do the same is a little bit scary.