If you want to use Darksteel, then I'd suggest using it as a weapon/armor template for gear/golems. Something along the lines of Darksteel-infused or something like that, with different levels of Darksteel used in the crafting process to improve the ability of the weapons/armor. Still, it would be a pretty expensive one.

Also, is it just me, or would Mirran grafts be totally badass? They'd be powerful, sure, but come with their own natural side effects. And maybe even corrupt you sufficiently into becoming a Mirran.

Otherwise, this is amazing. I can't even get started on how much I love this.