Give the Eldred decision, building new works on public domain ones remains problematic. I'd like to propose an alternative conceptual route.
In the word of digital media, the Copyright lobby's focus on copying seems very quaint. Making another copy of a work is trivially easy these days, so I'd like to suggest a modification to the idea of derivative works and the doctrine of first sale.
Derivative works should be allowed, as long as the full price of the source work is included in the price, and paid to the Creator of the source work (as long as the purchaser has not already bought it).
This avoids the whole commercial/non-commercial dichotomy that makes Creative Commons licenses so complicated, and has the nice property that works that derive from culturally widespread memes will be more attractive to fans. It is a neat answer to Ry Rivard's exponential growth issue and AKMA's call:
Discredit this decision by exercising our gifts and imaginations in public, without claiming exclusive prerogatives over those ideas� eternal destiny; share music, words, images, performances in person and online; but don�t make the mistake of thinking that Eldred v. Ashcroft closes the case on copyright.
I've discussed it in more detail at mediAgora