In my zeal to express How I Really Feel About Kingdom Rush, I neglected to address exactly what specific content I felt would make the game complete. Of all the suggestions I've made for Kingdom Rush, this one is the most important and has a greatest impact in terms of longevity for the series. And it's all about boosting replay value for the stages.
Our story begins in the form of creating challenges that uses the same stages and enemies, and therefore doesn't require creating a new engine or even new content. All that's really being modified is changing the conditions for completing a stage. The challenges I recommend are:
1. Max tower upgrades for earlier levels
2. Longer challenges for maxing all towers on all points
3. Multiple Campaign/Heroic/Iron challenges for the same stage
The above suggestions is what I feel is a good start for boosting replay value, and making the games "complete" in my opinion.