As far as I know, there are still plans to support custom textures, sounds, models, and maps at some point. A truly dedicated group of people could indeed conceivably replace all of the alien team assets with recreated Tremulous assets. Part of that work is already done: All of the unused Tremulous 2 assets (raw and/or UE3) are available for free use from
www.murnatan.com/history.
However as far as anyone knows, this is still just a planned feature and has yet to be implemented.
Server/client code mods are a more complicated story:
1) Modding - source code will not be released, it is not even possible and usable because of UE4 workflow (connected with assets, blueprints and plugins). But we want to have moddable game. This will be solved after release and we need to find best and simplyest way how to achieve it. Ideally you should be able to for example replace whole alien to different alien, that would be best - but I do not expecting that someone will do that, it is lot of work.
[...]
3) Creating maps - sure, creating maps will be possible. But UE4 is not compatible with anything, not even with UE3. So if you want to port any map then you have to completely recreate it in UE4 (or import it as a mesh etc.).
If you want to see an ATCS port made for Murnatan, you should talk to ViruS. Lately he has been posting on the GrangerHub Discord channel about his progress in making an ATCS port for Natural Selection 2.
