I think the main problem with Fable or Mass Effect was that the game wants to converge to one of a few endings, but definitely for ME there's a bajillion decisions you can make until you get there.

I don't know if you can get rid of this "definite" ending thing per se; some games say they have X amount of endings, but again, I can't really name any. It's probably more gratifying to have more self-contained sub-stories where the decisions made e.g. an hour ago have an effect on the progression and outcome, but not too much longer than that. You should have the choice as a player to switch from e.g. "good" to "evil" partway through your playthrough. References back to previous quests and their outcomes are nice but shouldn't be as heavy as "your one choice made 30 hours ago affect the ending of the game in a significant and irreversible way"