DISCLAIMER: There is nothing The Undertaker can do in 2017 that is going to be that entertaining. He's a terrible character and should retire.
With that said, of all the things he could be doing, I don't see why Reigns/Undertaker is being frowned upon by so many people already. There interactions so far have been good. Reigns isn't approaching this like any other "good guy" would and showing respect. He's telling Undertaker straight up that that the ring belongs to him now and essentially disrespecting a legend. I would surprised if Reigns doesn't win at WrestleMania and hopefully it will lead to his heel turn. If that doesn't happen, I will be kind of disappointed but I think it will. I don't think it has been anything special (obviously they have only had a couple of interactions) but it's far from terrible or worthy of getting spat on like some people are already doing. It has the potential to have a great conclusion too. Plus Reigns has been consistently putting on great brawls the last several months. He probably one of the best chances of getting a good match out of Undertaker that goes beyond the standard, "big move, kick out, big move, kick out, big move, kick out, etc.", but then again a lot of people fall for that shit.
The way I see it, if Undertaker has to be doing anything, then this is at least better than most of the other stuff he could be doing. I have no clue why people are interested in Cena/Undertaker, especially without the streak. That was the only reason I ever thought it would be a cool match-up - SuperCena vs SuperTaker. But there is nothing about how those two act that would make the story anything but shit. Reigns and Taker are just two badasses trying to stake claim to what they believe is their yard.