BlackFish wrote:1. I know it hasn't been done, but knowing that gzdoom supports 3d models, would it be possible to make an entire map as a 3d model
As a single model? Not a good idea. If the sector the model is in (since technically, it'll be in a single sector rather than in all of them, using the center of its collision box to determine which) gets out of sight, the entire model disappears... And, even if
this turned out to be accepted after all, it still wouldn't be a good idea, for a different reason: the entire model, meaning the entire level architecture, would be treated for rendering constantly; instead of having large parts of it culled out for being out of sight.
So, you'd get disappearing geometry and/or poor performances.
Brightmaps for 3D model skins is one thing I have never tried and can not say. I don't think it'd work, but maybe it would; depends on whether they're treated as textures or as something else.
Filling a map with 3D models is possible, though keep in mind the oft-forgotten limitation of needing the parent sector to be in sight. (It's a limitation that's also true for sprites, but it's much less of an issue for them, for some reason. People just don't tend to use sprites so much larger than the sectors they're in, or to animate them to be projected far away from their origin point.)