Mass Effect is a game with a lot of player choice involved but for me the king of player choice is the fallout series. From 1 all the way up to New Vegas it's massive, that amount of choice is where they should go with Mass Effect 4. However I will only be discussing Fallout's 1 and 2
Fallout 1: WOW! Is the only way to describe the choice, from wiping out entire towns to ultimately siding with the enemy you swore to defeat.
Fallout 2: This had everything a player could dream of. What other game allows you to be a slaver, a child murder, and slutty lesbian all at once? Sure you can say "that's not necessary" but that is the amount of choice the player had. You could even get in a war between rival families and become an adult actor / actress.
Point being even though a few things became canon later on they left the references vague enough ( in new vegas when they're discussed at some points ) to where it didn't override player choice. If Mass Effect 4 is a sequel that's what needs to be done. But what would they do about "Shepbringer!"?