Instead of a separate modding tool, I would rather suggest some sort of plugin, that could let you create in-game custom editors. Think of Age of Empires. The editor is actually a part of the game, and runs inside the game. With such a plugin, the game editor UI could also be modified to have the general aspect of the game . With a custom in-game editor, you could also set up certain specific stuff.
A separate modding tool is really not a good choice, because it would be received like some sort of Unreal Engine Free. This will fragment the community. I say no for external editors.
A plugin for creating integrated game editors seems the better way, since it could be adapted to have more game-specific features, easing the work of modders.