We have a base game almost ready to ship, we want to create some optional versions that come with downloadable extras instead of creating different versions of the game (dlc if you like) similar to map packs, how would this be done, we will be using the steam distribution network so it only needs to be able to read a local file but how could something like this be implemented?
If you’re using Unity Pro then you can use AssetBundle, though if you use in your DLC some scripts that aren’t shipped in the base version then you’ll still need to upload a new build of your base app (because scripts are not included in asset bundle, but recompiling the core app should add to it the scripts and then you can download the DLC as AssetBundle). If you’re using Unity Free then I think you can do almost nothing on this (I mean you could only recompile the full game and hide the content after your game logic).
You may want to read about asset bundles:
https://docs.unity3d.com/Documentation/Manual/AssetBundlesIntro.html