That's a good question
@dshirk
Now, even without the SDK there is a lot you can do:
- maps with the ingame WorldBuilders
- mapscripts with Lua
- XML mods for changing object (Civ, Units, Tech, ...) properties or add new effects using the available modifiers
- Lua/XML mods to change the UI
- Lua scripts for scenario making with a custom rules
- XML mods to add new Civilizations (with static leaders screen), new Techs, new Civics...
That would be much harder without the tools, but I think you can also:
- Edit and mix existing 3D models with ArtDef modding (which allows to create new units, improvements, features with pieces of existing models)
But you won't be able to
- upload your mod on the Steam Workshop
- preview existing and add new 3D new models (the later requires "cooking" assets with the SDK tools)
- debug your mod using the Firetuner (very useful for Lua scripting, also allows assets visualization)