We've learned more about Vince McMahon's decision to go in a different direction from John Cena vs. The Undertaker at Wrestlemania 33. The insight is completely from Undertaker's side and does not offer anything by way of the rumor of Cena working in a mixed tag team match for the possible retirement bout of Nikki Bella.
With Undertaker insisting before Wrestlemania 33 [instead of right after] that this will be his last match, Vince McMahon is actually [for the first time] taking him very seriously and has not bragged at all to those close to him [like he usually does] that he'll be able to change Undertaker's mind.
Undertaker's hip replacement is scheduled for after Wrestlemania and he's told Vince he'll never wrestle after he gets the new hip.
So as for the Cena match, it was Undertaker that went to Vince and asked if he could work a younger guy for his final match. It's Undertaker's preference to lose his last match and he feels the best thing for the business is to lose to a younger guy. Undertaker has nothing against Cena, but having already put over Brock, he was uncomfortable with the idea of putting over another one of the older talents and riding off into the sunset without ever passing the torch to someone from the new generation.
Roman Reigns is the new favorite to get the match. Vince did throw out the idea of Braun Strowman vs. Undertaker but Undertaker prefers Reigns.
Having written all of this, just because we write it's Undertaker preference to lose his last match, doesn't mean it will happen. Vince has the final say and what he says on show day is what will happen. We'll of course follow the odds carefully.