#1. Aleister Black

Aleister Black is a Superstar who has been benefiting big from his move to Monday Night RAW in 2019. Right off the bat, it was clear that he was a "Paul Heyman guy" backstage and started receiving a steady and consistent push that continues even today.
He's being positioned as one of the future top stars of the RAW brand and he has all the tools to make it big. While World Championship success still maybe a year or two away for The Dutch Destroyer, there are a lot of interesting possibilities for him with many Superstars.
He would make for one of the most exciting "new" opponents for The Undertaker and they would match-up well stylistically as well, as both Superstars are heavy on their striking game inside the ring.
One of the biggest things that could prevent this match from happening is how often The Undertaker appears. Given that his Saudi Arabia matches are normally against nostalgic opponents or Superstars who are set up to lose (like Rusev), WWE will have to go out of their way to book a match between Black and The Undertaker. One could argue that if Black continues on the track he's going, they could have an exciting clash at WrestleMania 37 next year.
However, it looks like it'll be a dream match that never happens.