Got several questions on this,
- You're hesitating between CC-BY-SA and GPL. From what I feel, GPL art comes from some oldchool projects producing art the free software way, maybe with some reluctance towards CC, or because it was non-existing at the time, because that license was shown to be efficient with code. CC is more popular in the sense artists, who do part of their job outside development/game communities, aren't really coders and don't have the same appreciation of licenses which were used for software source code.
Of course, you can't melt one with the other, but FPS asset productions has been using both, so ... I rather feel like you don't necessarily have to chose one or the other. You suggested they could use both, and keep things separate. I feel like it deserves the free software idea, which is: productions were made to be reused. The only drawback is you have to educate people not to melt these, but it's something you probably don't do often with art. For instance, a mapper could build a .map using textures from both texture sets.
Multiple-licensing is a possibility, but is it a need ? And what about a project which puts efforts into keeping assets license separated (in respect to history and choices made by past contributors from other projects), while using both ?
Second question is more about what kind of game iodoom3 could lead to. I haven't played Doom3, but I know the engine is notably used by Quake4 and ET-QW. It's maybe hard to get a view of that without trying, but maybe some guys who are already familiar with the engine could tell if it is more oriented towards one kind of game or the other. I suspect some answer like: making something like Doom3 is possible (see: the Dark Mod), while the engine is polyvalent just like ioquake3 was.
Your thoughts ?