Add to the notion of a Campaign. Arguably a campaign should be more than just a random collection of puzzles. There should be some kind of theme. So, have some form of Introduction to the Campaign come up when the user opens it, have some motivational message as part of asking the user if they wish to move on to the next puzzle upon solving the current one, and have some congratulatory message pop up when the user completes the campaign. Include some validation that what the user selects as a campaign file really contains a valid campaign.
New puzzles are always welcome, but they'll have to be particularly challenging to get extra credit.
Design a campaign around some theme. Make a proposal. Check with us before putting any serious work into such an idea, so we can tell you whether we think it's worth anything extra, and if it is, some idea of how hard it might be. This way you don't have to zip up your bitmaps folder, just your code and the new campaigns. Be sure to follow the directions for submission, as we won't have any time to track you down and get it right.