in Licensing Issues, Alex Schroeder asks:
Should I be happy that a Creative Commons license was used, or should I be frustrated because the license allows me to copy the map (and use it in my game) but doesn't allow me to easily build on it and contribute back to society?
This is my take on it.
If the author didn't publish under a CC license, then normal copyright restrictions would apply. Thus you have gained, so be happy.
CC licenses don't take anything away from you, and one of the things that they have left you is the ability to request further permissions.
So if you want to build on it and contribute back, then ask for permission to do so. They might grant it, and everybody wins. They might not, and all you've lost is the five minutes it took to write an email.
I've published under a Creative Commons Attribution Non-Commercial No-Derivatives before, and the reason I picked "No-Derivatives" wasn't to ensure that nobody every changed my work, but just so that nobody changed in a way that I wasn't comfortable with. I was open to requests for permission to create such works, I just wanted to know the author's intentions first.