#1 Mustafa Ali

Mustafa Ali (now just Ali, but we'll stick with Mustafa), is one of the most exciting performers the WWE have had in years, and given the history he's had with Joe in 2019, this makes the most sense from a continuity standpoint.
From a pure babyface vs. heel standpoint, this is also the best match that WWE can put on under the unfortunate circumstances that Mysterio can't compete, and it's likely the WWE's #1 'Plan B'.
Sting in WWE once again? More details HERE
Last year, Ali competed on the pre-show against Cedric Alexander, but his star has been constantly on the rise ever since that point, so a match for the United States Title is much more appropriate that the pre-show battle royal he's currently booked for.
It seems a little too early to take the title away from Joe, and given that Kofi Kingston, Becky Lynch and Seth Rollins could all be crowned on the same night, they'd likely not do the same with Mustafa, but it would still make for an amazing match.