#1 Brock Lesnar
Brock Lesnar is the current Money in the Bank briefcase holder, which guarantees the Beast an opportunity to challenge a world champion (WWE Champion or Universal Champion) at any time or any place. Lesnar's been playing mind-games with both champions, and he has teased cashing-in on many occasions but hasn't cashed-in officially yet.
Seth Rollins seems to be Lesnar's target, although he could always cash in on Kofi if he chooses to. Rollins' feud with Lesnar is far from over, and Lesnar still hasn't forgotten WrestleMania, where Rollins defeated him to win the title. If Lesnar decides to show up, he would definitely play a factor in the Universal Championship match.
Baron Corbin has to choose a special guest referee. Who knows who it could be? It could be anyone from Triple H to Heath Slater (although that's pretty unlikely). It could even be Lesnar. Even if Lesnar isn't the special guest referee, he could always cash in his MITB contract for a Universal title match.