#1 Should: Pentagon Jr.

Pentagon Jr. has been treated like a big deal in every promotion he has wrestled in.
He's a former Lucha Underground World Champion and Impact World Champion. His work in Lucha Underground since its beginning is what has helped build that promotion into a believable, alternative outlet for those looking outside the WWE.
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He's like the Mexican Stone Cold, not caring what anyone thinks or wants. He'll fight one guy or 10 guys and is willing to fight in any match.
Seriously, watch his Grave Consequences match, Ciero Miedo match with Vampiro or his Gauntlet Match with the likes of Kairi Sane and Io Shirai before they signed with WWE. They are amazing.
He's also an amazing tag wrestler. My only hope is that if he signs, it's as a singles wrestler. He and Fenix are likely going anywhere together, so they need to be signed as singles wrestlers.
When feuds necessitate them being a tag team, then they can tag together. He's one of the most popular wrestlers on the planet, has a built-in character and could be another anti-hero for WWE if booked correctly.
WWE is still looking for its next big Latino draw, and if signed, he needs to be treated as big as AJ Styles eventually. He's that good.