One way to make the game continue to be interesting would be to have a "Random Level" option. Here's what I'm brainstorming:
1. QuickPlay menu has "Random Level" showing, looking like another campaign. This might only be available after you have 3 starred X number of non-challenge levels.
2. User gets to choose "Normal", "Hard", or "Insane".
3. Random level is selected from all non-challenge levels that the user has 3-starred. A few of the earliest levels might be excluded here.
4. "Normal" gives the user the same experience as if he choose this level from QuickPlay himself, but he doesn't see the name of the level. "Hard" fills one of the slots with a random defense this user has unlocked. For example, one of the Dungeon levels might have Jelly Cube filling a slot, so that the user can't remove that Jelly Cube and has a choice of either planning a strategy that includes Jelly Cube or else a strategy that uses one fewer slot than normal. "Insane" fills two of the slots with random defenses.
We would probably want to have some random defenses not count, such as mana crystals. I don't think that we could use 3 random defenses, the way that replaying adventure mode in Plants vs Zombies works, because there just aren't enough slots - unless the user just clicks for a different set of random defenses until he gets a set that he is happy with.
At least part of this idea stemmed from the fact that I have noticed I tend to choose mostly the same defenses every time, but I don't like being told "no you can NOT use your lightning tower!", etc.