#4 AJ Styles vs. Kevin Owens for the United States Championship
Kevin Owens and AJ Styles are two of SmackDown's biggest stars and would seemingly be prime candidates to take the WWE title from Mahal, but they're still in somewhat of a midcard rivalry involving the US title.
Unless a midcard title is being held by someone like John Cena or Roman Reigns, WWE is typically hesitant to have a mid card champion also feud for a world title. Right off the bat, that eliminates both Styles and Owens as potential contenders for the WWE Championship for the foreseeable future.
SmackDown has a rather thin roster, and with Styles and Owens settled in the mid card, Mahal should feel a lot more comfortable with the WWE Championship. After all, WWE seems to be building the US title back up, which means that several potential main eventers who could be challenging Mahal for the title, may be challenging Owens or Styles instead.