#2 He'll probably be a part-timer
If CM Punk does return at the Royal Rumble, he will end up winning it - hands down - unless WWE plans to provide us with a major swerve. He might also go on to dethrone Brock Lesnar at WrestleMania to become the WWE Champion, but let's take a moment to realize why so much of the WWE Universe doesn't approve of Lesnar as champion in the first place.
No points for guessing, it is because of Lesnar's limited appearances as he only shows up on major occasions and events, while often missing PPVs like TLC. Will having CM Punk dethrone him solve that issue? Mind you, CM Punk is 41 years old and hasn't had a wrestling match in nearly 6 years. There is a high probability that CM Punk might just become another part-timer who shows up for major events and PPVs only.
When 'The Beast Incarnate' broke CM Punk's famous 434-day World Title reign, fans instantly began comparing the two. However, there was a huge difference and that was in the number of times each Superstar defended their title. CM Punk was a fighting champion that defended the WWE Championship with regularity, but Lesnar - not so much.
It would be heartbreaking to see CM Punk return, only to become that kind of champion.