I rather suspect the Reapers' sudden return might have something to do with those stars behaving strangely and emitting dark energy readings. Perhaps an ad hoc version of a Mass Relay or some other even more advanced technology they left behind as a redundancy.
Not sure if I like the idea of multi-player being tacked on.
That's my assumption, as well. All the talk about dark energy and stars aging faster than they should sounded like setup for ME3.
And I hate the possibility of multiplayer in ME3. If they want to make a multiplayer game in the ME universe, fine, but leave it out of the main games.