Mass Effect is more than him/her.
I agree. If they did a prequel you could basically have a game that starts with the First Contact War like for the prologue/tutorial section then skips ahead 5-10 years.
This could be the game that Andromeda wanted to be. Humanity is the fresh newcomer to the wider galaxy and they only started colonizing other worlds.
The story would have you scout new planets, build colonies etc. while always having the option to visit the Citadel, play politics, fuck the aliens.
This would be a smaller story, no saving the world, but building a community, securing the space for human colonization.
The consequences would stem from what would happen to the colonies you built personally not from what Reapers will do the the galaxy.
We already know that other colonization efforts succeeded but we wouldn't know what happened to the bunch of colonies you established yourself.