This is the perfect Obsidian game to do this for. You're using your own engine, so you get to code the tools and stuff yourself. The game has lots of facets to work with (levels, items, classes, NPCs, monsters, etc). And finally modding adds significantly to the lifetime and reception of your product. It allows fans to mould the game to their liking, patching out what would otherwise annoy them, helping word of mouth, etc. And it allows players to come back to your games years latter to try out the latest improvements or unofficial expansions - which is always a good thing, because the longer people play your game, the more copies it will sell.
So come on, help us help you Obsid! Even if you just make everything soft coded in easy to decipher and read file formats (like the Infinity Engine games largely did) - that'll be enough for the hardcore modders to build and release tools around.