Saveable assemblies in the Map Maker, save terrain presets

I've just started poking around at making maps and have quickly come to the realization that my computer can chug a little while running larger maps. So for the sake of experimentation I opened a small 50x50 map and built some things in it. I think it would be an excellent feature if we could select an area and save it as an item we could then rotate and place onto different maps. Essentially creating custom terrain brushes.