1 - WrestleMania Main Event
The one goal that Punk was never able to accomplish was competing in the main event of WrestleMania, and because of that, he looks at his WWE run as a failure. However, many would argue otherwise, as he ended up being the most popular superstar in the company even though he was never truly presented as WWE's number-1 star.
As for who he'd face in the main event of WrestleMania, well, the best prediction is Roman Reigns, simply because he's been in the main event of the last four WrestleManias, and on top of that, a Punk vs. Reigns match on WWE's grandest stage would generate a ton of interest, and the match itself would have a tremendous amount of heat.
A female WWE star said her life is in danger. Details HERE
Another idea and one with serious doubt that WWE would do this; to have Punk main event WrestleMania against Daniel Bryan in either a Submission match or an Iron Man match. But if WWE were to book that match upon Punk's return, it likely wouldn't end up being the main event of their biggest show of the year, but it would more than likely be the main event in the fans' eyes.