#1. AJ Styles vs Shawn Michaels

It's kind of like when The Beatles broke up. Ever since Shawn Michaels retired in 2010, everybody began asking aloud - when is he going to come back for one More Match? And, every time, Michaels would say "I'm not, stop asking." Let's consider for a minute that AJ Styles wound up in WWE in 2010.
Michaels had his retirement match with The Undertaker that same year at WrestleMania, and used the opportunity to hang up his boots, as he felt he had done everything he was going to accomplish in the world of pro wrestling. And, let's be fair - that was a lot of stuff.
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On the other hand, he had to be aware of who AJ Styles was at the time and the reputation he had around the rest of the business. It's not a stretch to assume that Michaels might have been willing to extend his time in WWE just an extra year for the opportunity to work with The Phenomenal One. After all, it's not like The Undertaker was going anywhere.
In 2017, Michaels admitted that he was asked to face AJ Styles in a match that year at WrestleMania 33. Shawn, obviously, turned it down and explained on The Sam Roberts Show:
“I could’ve had that match, they did not have an opponent for AJ at Wrestlemania. I wish that man was here 10 years ago man, I really do.”
Now, that was in 2017, meaning that his own dream match with Styles would have taken place in 2007. However, it's more than likely that someone with the reputation of one AJ Styles coming into WWE in 2010 would have given Shawn the spark he needed for just one more run before calling it quits.