Mass Effect was heralded as a franchise where individual decisions players made would reverberate across the entire video game trilogy as Commander Shepard leads an alliance of civilizations from across the galaxy to drive back the Reapers, an invading race of sentient machines. There are plenty of choices players make in the earlier games that lead to bigger consequences as the story of Shepard and his friends progresses but one major decision that Shepard can make at the end of the original Mass Effect is quickly brushed under the carpet by Mass Effect 2 and virtually inconsequential by Mass Effect 3 making something a plot hole in the trilogy should this option be made.
At the end of the original Mass Effect, the Reaper vanguard Sovereign mounts an attack on the Citadel, the capital of galactic society and focal point for the incoming Reaper invasion. Shepard is given the option to commit the human fleet to intervene to take down Sovereign, saving the ruling council, albeit with heavy casualties, or to deliberately hold back the fleet and allow the council to die. This choice can be compounded with Shepard placing Ambassador Donnel Udina to represent humanity on the new council and conspiring with him to install a predominantly human council to rule over the galaxy and ensure Earth's best interests are prioritized over other civilizations.
While the decision appears to be a cataclysmic one on the surface, showing just how far a renegade Shepard would work with Udina to steer the galaxy's ruling body in advancing humanity's agenda, it's a decision that is forgotten almost as quickly as it's enacted. While Udina remains on the council straight through Mass Effect 3 -- even taking the council position by the third game if Shepard selected Admiral David Anderson to assume the role instead -- the council still features a mix of representatives from different civilizations by the start of Mass Effect 2. To make this oversight all the more glaring, a renegade Shepard will react with shock and surprise when Udina betrays the alliance to Cerberus to forward humanity's agenda during Mass Effect 3 -- despite literally being present when Udina expressed such intentions in Mass Effect's conclusion.
While the council plays a relatively minimal role in Mass Effect 2, with a resurrected Shepard and his crew tacitly working for Cerberus and the Illusive Man to stop the Reapers, a predominantly human council would have significantly impacted Mass Effect 3's story. Mass Effect 3 is a game about humanity earning the trust of all of the other civilizations, convincing them to rally and repel the Reaper invasion which has been using Earth as a staging ground. Shepard and Udina's decision to install a primarily human council to run the galaxy would likely have come back to haunt them in this scenario, with the other civilizations more distrustful of humanity than ever.
Regardless of the perceived impact of player choices, the Mass Effect trilogy has been thrilling players for the past fifteen years. Still, the option to work with Udina to create a human council to run the galaxy is a curious one if it is just as quickly dismissed by the time the first sequel comes into focus and examines how the Citadel has changed in the two-year jump between games. In the end, Udina was always going to represent Earth on the council eventually before betraying them to Cerberus but the council itself would never maintain the pro-humanity angle a renegade Shepard and Udina could've been vying for.