Apart from the ones related to units (models, textures, sprites) and the UI related ones (which end up being replaced in most mods anyway), the rest is mostly required, so it's not worth th trouble trying to find out exactly which are not. I tried pruning FATW's modfolder from the unnecessary files, and I ended up removing some 5 MB of stuff, which is nothing compared to the total amount of files. A mod that doesn't use some cultures will be able to save some more space by removing the battlemap building stuff that is not necessary. The most trimmed down modfolder of a BI-based mod that I know of is the VI2 one.