The Undertaker is one of the most respected men in pro wrestling. AJ Styles is one of the best in-ring performers right now.
Both have had great careers, especially Undertaker. His thirty-plus-year career in pro wrestling has been a cornerstone in WWE since his debut in 1990. He's had multiple title reigns, various incarnations, and a catalog of great matches (most recently against Shawn Michaels at back-to-back Wrestlemanias). Styles, on the other hand, made his career in TNA and New Japan before making his long-awaited tenure in WWE (winning the WWE Championship twice already). His rivalries with John Cena, Daniel Bryan, and even Kevin Owens have been some of his best work in the company...so far.
It's simple: AJ Styles must face Undertaker to solidify himself as a true WWE great.
The two could easily work together to tell a story. As soon as Styles comes back from his short injury sabbatical, he could come out and say during that time, all he could hear and see was Undertaker matches at Wrestlemania on TV and the internet. He'd say he's sick of hearing Undertaker's name and is glad Undertaker has finally left the company. He'd finally take a second to think and offer something to Undertaker: face him at Wrestlemania. WWE could do anything to build up the match, but make it sensible and logical. If it were just a simple 'dream match' that could spawn into a post-Wrestlemania rivalry/program, then maybe that's okay. If it's just the match and only the match, no worries. Either way, give the duo time on the card, and you got yourself an awesome Wrestlemania match.
Despite some fans wanting something else, I think WWE planning Undertaker/Styles at 'Mania this year is great. It could give Undertaker a really good final match against the best of the company, and it could be Styles' best WWE match so far. I'm looking forward to see what does happen, but if the rumors are true about putting the two in a program together, then perfect.