#3 Kenny Omega
Reportedly, Kenny Omega's contract with New Japan Pro Wrestling expires in early 2019. When it does, WWE is expected to make him a big money offer. If for whatever reason a full-time contract doesn't get signed, though, the company might wish to consider offering him a per appearance deal.
Omega's presence would generate a ton of buzz regardless, and a per appearance deal wouldn't overexpose him. Instead, any time he came back, his angles and matches would have a big fight feel, though obviously, he would need to make more appearances were he to ever win the WWE or Universal titles (the last thing the company needs is another Brock Lesnar situation).
Even without WWE's umbrella, there's arguably no hotter wrestler in the world right now, and if he doesn't want to go to WWE full-time, a per appearance deal would give him and his fans the best of both worlds - a bigger spotlight and the freedom a full-time deal doesn't convey.