Lockboxes. They all have excellent heroes in each set (other than Constrictor imho), but they can cause great agony. Some sets can take you over 13 duplicates and others less than 5.
The thing that bothers me the most is that you never have the slightest idea how many dupes it would take. It should really have system that assures you that with a specific amount of lockboxs you can get the hero.
The ability to guarantee one new comic cover with 30 lockboxes. Once you get all 8 covers for that particular set this method will be locked.
Once you open 250 lockboxes for a particular set, all of the covers you now open for that set will be a new cover guaranteed, until you get your 8 covers.
The ability to purchase any brand new cover for 50 CP. (from ArkadyDarell's comment)
In the comments, type which one of these changes you want to see in the game. Also type in the comment s if you think the lockbox system should stay the way it is.