Has anyone ever considered making a tyranid mod? I would imagine it would be too hard. There is plenty of images for textures, we know how they fight, etc etc. Making a brain called the "norn queen" with a magic "gun" that spawns creatures would be cool. have a big radial bar that lets you select a creature type, and then that spawns a new gun which allows you to choose special modifications for your swarm. You could create rippers to consume materials in the ground as well as fallen corpses. (Have the materials people are made out of, like metal and flesh, yield gold) and spawning units drains your gold, or "biomass." Only the larger units that are synapse links would be irectly controllable, with gaunts simply set to brain hunt ai mode.
Just an idea, I am willing to help with indexing and neatening, as well as basic guns.