If the reapers did harvest other galaxies besides the milky way, this would make for an interesting power vacuum in many galaxies as the reset switch is no longer present. I guess it would also mean that the reapers were not in sleep mode during the time between cycles in the milky way. But until there is some sort of stationary relay revealed, this is just a shot in the dark and speculation on my part.
The Prothean aspect would be an interesting twist as well, but if (Big If) the reapers did harvest other galaxies and built relays in them I wonder how many other powers took the prothean route as we have seen in the Javik story line and actually pulled it off. I mean having a section of their population hiding with its peak level of technology from before the harvest would provide for some interesting and powerful enemies.
So far the only thing that seems to be present is the Andromeda galaxy is in a dark age type description in regards to galactic civilization, and I like many others have just been trying to figure out and speculate why this area is in a dark age.