In another game I play (puzzle, not tower defense) you have to solve each puzzle in a level pack in order, just as you do in campaigns in TH. However, you can spend 99 cents to purchase a "Master Key" which allows you to play the puzzles in any order you like, so that if you are stuck on #12 of a pack of 20, you can still try #13 etc.
I would think that this would be easy to implement (depending on how keeping them locked is implemented of course). I for one would be willing to pay for this for Tiny Heroes, because there are some (early!) levels of Gauntlet and Carl's Funhouse 2 that I just can't move my hands fast enough to get even one star, so most of the levels in those campaigns I'll never get to even try.