Replacing Nonfree Assets

[tempest]

The Cube engine is written under the GPL license

No. Zlib-like with an added sort-of-copyleft clause:

Permission is granted to anyone to use this software for any purpose,
including commercial applications, and to alter it and redistribute it
freely, subject to the following restrictions:

1. The origin of this software must not be misrepresented; you must not
claim that you wrote the original software. If you use this software
in a product, an acknowledgment in the product documentation would be
appreciated but is not required.
2. Altered source versions must be plainly marked as such, and must not be
misrepresented as being the original software.
3. This notice may not be removed or altered from any source distribution.

additional clause specific to Cube:

4. Source versions may not be "relicensed" under a different license
without my explicitly written permission.

My main proposal is that an initiative is launched for the community to provide GPL-licensed art

No, please. If you do a relicensing, choose a sensible license. How sensible the GPL is for code is disputable, but for art it's really not a good idea. Of course, I don't want to tell you what to do (if you find enough GPL-licensed replacement stuff, thats fine), I'm just stating my opinion.

The crosshairs are a non-issue, they're all CC-by-sa. Many sounds are also under some free license.
One big issue are maps. Next to none are freely licensed, and they're (obviously) very hard to replace. Another are textures - most are under varying proprietary licenses and would be hard to replace as well.

Either way, specially given the current state of AC development, there are more pressing issues. The thing is that most people care more about having good artwork in a good game than having artwork that complies with the licensing guidelines of some linux distros. Plus, the stuff can almost always go in -unfree anyway.

Notwithstanding, if people actually manage to find equivalent freely-licensed replacement artwork, that would be optimal, and I'm sure everyone would be happy about that.

Lots of open source games, especially popular and successful ones, do this. It's usually to prevent forks, because there are always community splits on ideas and such.

Not really, no. Often finding freely-licensed game art is a real challenge (although it got a lot better in the past few years with OpenGameArt, BlendSwap etc.). Also, some stuff (e.g. Quake textures) was made and published when Creative Commons licenses didn't even exist.