#2 Brock Lesnar

When looking back on all of Kurt's WWE rivals, none brought the best out of him like The Beast, and despite the physicality Brock provides in the ring, it's an option WWE is definitely going to consider.
Their Iron Man match on SmackDown is still considered one of the best televised matches the WWE have ever done, and if they could both still go like that, Brock would be the only choice, but unfortunately, time has diminished both of their skills.
Did you know a top WWE Superstar just wrestled outside the company? More details HERE
It's clear that Brock himself is also on the way out of the WWE, whether it be this year or next, and if they look to push the future stars in the company as champions, this would be a great way to keep him around.
Ultimately, this is likely the way WWE will go, considering what they've allowed Brock to do over the last 5 years. And given the history, it's probably the right choice. We just hope that Kurt isn't injured, given all the German Suplexes Brock likes to give.