A good example is the Learned et al's recent paper on The Cepheid Galactic Internet, arguing that aliens would use modulation of Cepheid variable stars to encode messages to other civilizations. The Dyson Sphere is an older example. A more recent one, Hsu and Zee's suggestion that the CMB contains a messages. Various papers on warp-drives, wormholes and time-travel would also qualify. What do you think?