#3 To set up a champion vs champion match at Royal Rumble
There have been rumors floating about that the World Championship feuds will spill over to the Royal Rumble. Brock Lesnar's next title defense will be at the Royal Rumble 2020 since he never wrestles at TLC and always appears for the Big 4 PPVs.
Mysterio presents a good opponent and fans won't have any problem with WWE running the rivalry back. Mysterio would most likely lose again if they faced off, but it would be interesting to see them have a rematch but with a whole different story.
Check out which wrestler EC3 finds cute RIGHT HERE
If Mysterio is still United States Champion by January 2020, then it's perfect as it sets up a Champion vs Champion match. Having Mysterio as an active champion facing Brock Lesnar would sell the match more and perhaps generate even more hype than their match at Survivor Series had.
Royal Rumble doesn't seem like the place you would usually see a Champion vs Champion match, but it's fine since the United States nor Intercontinental Championship are regularly defended at the PPV.