Cool—sounds like a fun post.
At this point SETI can rule out some hypothesis classes—namely those involving colonization of the galaxy by bright hot civilizations. (energy limited stellar based civs)
However, from a pure future engineering perspective, the cold dark model for future civilizations seems more likely on first principles (temperature constrained civs). A galaxy populated by cold dark ET civs would more or less look like what we see now. Perhaps some future clever technique could detect cold dark ETs, but at the moment it seems difficult short of sending out a ton of probes and waiting centuries—probably faster to just create superintelligence on earth and use that to figure it out.
I’ve written a little about the cold dark model and the ‘fermi paradox’ - here.