Spoilers warning: I'll be spoiler tagging any Mass Effect 3 spoilers, but anything from ME/ME2 won't be spoiler tagged. You've been warned.
In the Mass Effect universe, the primary means of interstellar travel is by a system of Mass Relays. Faster than light travel does exist, using similar technologies based on mass effect fields, but the relays are much more efficient for interstellar travel given the immense distances between inhabited systems in the galaxy.
In the "Arrival" DLC for Mass Effect 2, we actually see the destruction of one of the relays. Also, in Mass Effect 3,
the beam careated by the Crucible and the Citadel in the game's final sequence destroys (presumably) all of the mass relays throughout the galaxy.
The races of the galaxy have been studying the mass relays and using mass effect technology for hundreds of years. Humans have been examining and using the beacon on Mars, which originally led them to the Charon relay itself. In Mass Effect 3, we also discover that
the Asari have been gathering information and technology from a Prothean Beacon for an unknown amount of time, providing for their technological advancement.
Given the familiarity of the races with the mass effect fields and the relays themselves, what are the chances of a relay, or relay system, being built by the races of the galaxy? This would obviously be no small undertaking, but is it ever commented on? Has it ever been tried?