yeah mods change js components and easily modifiable things like damage values and other properties. Core engine components need to be compiled (c++ files) or depend on things that need to be compiled and therefore cannot be applied as a mod.
Once added to the main game, we could, however, modify the weight values and other things @Atrik has set up in the unit templates. And that could be nice to test adjustments in the community mod.
I don't know what mods can possibly do (never dealt with them in any game). I assume you mean that they can't change the engine, and that's why this can't possibly be part of the community mod?