What do you think about getting some kind of trophy/plaque/souvenir for your SG Base when you complete a TF? Like the big penny and the T-rex in the Bat Cave. Not like the display cases you could buy, but an award to show you actually completed it. They wouldn't have to be gigantic, and you 'could' buy cases or pedestals, etc. to display them on. Oh, or maybe you could scale them, so they COULD be gigantic!
As an aside, if we get personal lairs, maybe news clippings or something for completing mission arcs.