#3 The 'special' feeling could wear off quickly
Depending on how WWE decides to bring him back, the run could be pretty short. If Punk comes back as a full-time performer, even if it's just full-time on TV with a limited house show schedule, he could potentially become just another cog in the machine before we even realize that it's happening.
For example, if he shows up at Survivor Series in November, which is taking place in Chicago, it's possible that the roof could actually blow off of the arena. But what comes after that? Explaining his actions the next night on RAW or at the end of the week on SmackDown would be thrilling. His first match back would be one of the most highly anticipated matches in years.
Post that, he's another guy on the roster.
It may take six months or more for the shine to wear off, but when it does, it's very possible that Punk could be a guy meandering around in 6-man tag team matches a week before a pay-per-view that has three announced matches.
Is that a definite? Of course not. In fact, if WWE starts moving heavier in the direction they appear to be going, maybe things like that won't happen at all. Positive changes are definitely coming to WWE, but for CM Punk to be one of them, it's probably best for him to show up just a couple of times a year. More matches than Brock Lesnar, for example, but not matches every week like AJ Styles.