You should poke around the maps directory, both in the dgdata.zip and the bindata folder, all the stuff you're going to need to make is in there for the official maps.
Key files:
MAP_XX.scmap : This is the file containing the heightmap, and is probably similar if not identical to the one used in SupCom, there may have been an editor available during beta judging from what is in the threads over at GPG. You can think of this as the same system as a game like age of empires, where a block of land is determined to have a certain height (probably a float in this case), with some maximum indicating impassable terrain. DG doesn't have any maps using multiple heights, but there is the possibility that it could if it is using this same system (I looked at your map suggestions and I'm guessing you're trying to make on of them).
This file will control where units can and can't go.
MAP_XX.gr2 : This is the mesh for the level. In all the official maps they are complex multi-part 3DSMax 9 creations. If you want to take a look at them to get an idea of scale, get the Granny Viewer (google it) and open them up. I am not responsible if you go insane bitching at how shit this program is.
You have 3D modelling experience, which means you're exactly the type to be making new maps, unfortunately as you would know, you can't export to gr2 from 3DS (or in fact any program for less than $10k) so you need a plugin to do that. Since gr2 is closed format, your best bet is to try to find a granny2.dll that is the correct version for demigod - 2.7 - and try to use it with obsidian's expotron.
http://gr2decode.altervista.org/
I haven't been able to find a new version of granny2.dll, the one in my nwn2 directory is too old, DG doesn't come with one, and I can't be bothered installing every game I own from the past couple of years to check them for it.
But there are more problems, if you look at Chirmaya's effort, there are issues binding effects to meshes, and probably with binding the 3 texture maps to meshes too. I have very little 3DS experience (I code, not much else) so I'm not sure how you would do this normally. I don't have the scmap editor installed, but looking at the scmap files in a text editor there are also .dds (DirectDraw Surface) textures referenced there. Poking around inside the gr2 files in the viewer you can inspect the values for a large number of structs, none of them seem to reference effect files, so I'm not even sure how that particular system works.
I've probably made false assumptions along the way, maybe its all simpler than I'm making it out to be. But ultimately it's a mess, I toyed with the idea of trying to piece all this together, then decided that it's really not worth spending my vacation working on since it may be unsolvable and it's the sort of thing GPG should be doing for us anyway.
If you wan't some help, I suggest you try to contact Chirmaya on IRC or through the forums. Get a feel for the dimensions by looking at the gr2 files, and start making your stuff in 3DS, and hope GPG come out with a gr2 exporter sometime soon.
MAP_XX_blahblahblah.lua : There's a whole bunch of lua files in the dgdata.zip map directory that control creep paths, building locations and spawn times (linked to war rank) flags, rules etc. You can really easily stuff around with this stuff without breaking anything.
