#3 Kane
At No. 3 on this list, we have The Undertaker's very kayfabe brother- The Big Red Machine ie. Kane.
As we are emerging into the end of The Phenom's wrestling career, another realistic opponent for him would be none other than Kane. This is a match that many fans have mixed opinions on, but it's certainly a match that would make sense in terms of storyline.
We saw The Brothers of Destruction reunite at Crown Jewel 2018 where they went head to head with D-Generation X in a tag team match that received negative reviews from critics. This match came to end when Triple H hit Kane with a Pedigree to gain the win for his team.
Kane and The Undertaker have battled many, many times before in the WWF- as their feud is known to be a very historic rivalry that old school wrestling fans will remember for a long time. As a matter of fact, The Undertaker was Kane's very first rivalry in the WWE and it was revealed that the two men were storyline brothers.
One possible reasoning for this feud could be because of The Brothers of Destruction's loss at Crown Jewel 2018- as Kane could turn heel by blaming The Undertaker for their loss and not doing enough to help him during their match against DX before challenging him to a match at Wrestlemania 35.
We have seen Kane and The Undertaker compete before at Wrestlemania 14, in a match where The Phenom defeated The Big Red Machine.
But we've also witnessed Taker have WrestleMania rematches with Shawn Michaels & Triple H before, so it's not unlikely that this match may occur.
Despite the fact that Kane is currently the Mayor of Knox County, Tennessee- we did see him return for the huge tag team match at Crown Jewel 2018, which means that Kane could easily make another return to the WWE for a final rivalry with his brother.