The second most chosen candidate is Chris Jericho. This is what I like to call the "Sting Choice" where everyone is trying to find clues such as using maths on the dates to turn it into "Y2J" which for the record is an expired gimmick which I highly doubt he will revisit. The promo just did not fit his character in anyway and I hope people don't get too disappointed if it does not turn out to be him.
Y2J is not an expired gimmick, it's a monicker that will always apply to him and a nickname that's stuck with him his entire WWE run.
The promo fit his character pretty spot-on, as it was mysterious, featured extensive vocabulary and used a Biblical quote, all of which applies pretty heavily to Jericho. Another thing people don't seem to acknowledge is the tone of the video.
Look at any Undertaker return video. Before Judgement Day 2000 (when he returned as the American Bad Ass), before WrestleMania XX and last year... One thing that is common in all his videos, regardless of his gimmick, is that they're dark. He's the Lord of Darkness, his return videos have taken place in dark basements, graveyards, nightlit shacks... Never anything brightly lit. Not once, ever, in the history of his character. And this whole video was bright. The schoolroom was bright, every outside shot was during the day. It doesn't fit his history at all.
Furthermore, what would he have to come back to take that's his? Titles are pretty much out of the picture, he can only wrestle one match a year at this point because his body's so broken down. Everyone pretty much figures WrestleMania's gonna be his last match anyways. He already has his streak, he doesn't need to come claim it, someone needs to challenge him for it. That will probably happen around February again, 'cause he just doesn't have the longevity anymore to keep something going for 3 months. And why in the hell would his return bring about "the end of the world as we know"? That's a pretty blatant heel line and promo, something a bad guy would say and do. Undertaker's not going to be heel going into his 20th WrestleMania win. No one would boo him.
Chris Jericho IS the most likely candidate. WrestleZone themselves reported that WWE was in talks to bring him back around the Royal Rumble, he made his Twitter comment to throw people off and he's expressed extreme discontent with CM Punk for stealing his "best in the world" title. The video is laced with Jericho-like metaphors with its wording. He's coming to take back what's rightfully his and play a brand new heel Y2J to go into WrestleMania. For both his debut and his return, he said he was coming to save the WWE. And now he's coming to end it. That is literally the simplest option, not the Undertaker.