#2 Braun Strowman vs Roman Reigns
Another scenario where by Strowman could cash in his briefcase would be against Roman Reigns. Should Reigns face and defeat Lesnar to become the Universal champion, Strowman could cash in his briefcase and beat Reigns for the title.
This would surely be an enormously well received angle given the WWE audience's disdain for Reigns, particularly as far as holding the top title is concerned.
This could also set up a feud between both men and possibly begin to position Reigns into a heel persona up against the fan favorite Strowman.
Strowman could decide to wait for his opportunity to face Reigns, perhaps teasing a cash in on Reigns after he defeats Lesnar, building towards the eventual moment when he does cash in.
There is the possibility that WWE could pull a swerve and have Reigns beat Strowman, leading to Braun snapping and annihilating The Big Dog, positioning The Monster Among Men as a heel and Reigns back as the sympathetic face.
Whether the WWE audience does decide to get behind Reigns is hopeful at best, but long term rivalry between the two with Strowman as the heel could make for some thrilling TV.