Since the only possible prequel with a human protagonist( pretty sure that part is locked in) would be the First Contact War, that won't happen. Nobody wants it. And Mass Effect is just a little too serious for any kind of time travel story. As for an alternate universe, I don't understand why anyone would want this. It would basically be wiping the entire lore slate clean. Bioware likes their lore just a bit too much for this kind of thing.
Now, regarding sidequels. A game telling a story parallel to the events of ME:1-3. I haven't seen anyone discuss this yet, and that is probably only because I haven't looked very hard, but there are new species confirmed for the next game. At least two as of N7 day. As far as I know there was only one species, detailed in the codex, that is not featured in the games, the Raloi. Now they could easily have added that species to the new game, but that leaves us with another. Bioware could easily pull something out of their back pocket, "Oh this race was just living in secret the entire time," or whatever. But the fact remains, you can't go adding new species that were supposedly on the galactic radar as of 2186 and just saying, well deal with it. The other scenario would be, something I think people have talked about, a group of explorers setting out into uncharted space around the time of the Reaper war, and coming across new species on their journey. This satisfies most of the requirements to make a sidequel with the properties we know the next game has. That would actually be something I wouldn't mind getting into. The only issue is, I feel like it is just running away from the first three games instead of acknowledging that they happened and moving on. I think, and hope, that Bioware has put enough time and effort into building their version of the post-war galaxy. It may not give everyone what they want with regards to your specific ending choice, but who cares? We are way too spoiled when it comes to that kind of thing. What we need is a great game, and if they can deliver that, I'm all for my choices having less long term inmpact in the next title.
Went a bit sideways there, so to sum up:
Prequel= boring, no one cares about the past.
AU= kind of just throwing out the majority of the universe (history, characters, events, etc.)
Sidequel= Interesting, but at the end of the day, will still leave people saying, "Yeah, but what happened after the Reapers?"
Sequel= Lot's of people are happy, some people are angry. Just like every other game/movie/story in history.