I can't account for the racial homeworlds, though the Citadel? I can almost guarantee it making a return, for reasons I am sure all of you will be able to understand. Additionally, just a bit of a rant from me: why do people keep proposing travel to a completely new galaxy? Why would anybody want that? I can barely understand beyond the concept that being in a new galaxy will just give us different colours to look at, and that it would make Mass Effect something completely different; you'd may as well just give it a new name to begin with.
Bioware definitely has to stick with the Milky Way, and that they shall. Besides, how could you possibly grow bored of an entire galaxy? There are billions of systems and planets to explore, the possibilities are endless! So new galaxy? Waste of time, and will gain more hate than love when people are torn from the universe that they know and love. I suppose one way to make a new galaxy enjoyable would be to have the races of the Milky Way perfect technology to such an extent that they craft travel to another galaxy, and meet a whole array of new species, but that would be a helluva lot of work.
Speaking of new species, I hope to see Bioware give us a few more in Mass Effect 4. I'm assuming they will, because with exploration at such a grand scale (or so it would seem), they can't just rely on Varren, Thresher Maws, Pyjaks and Space Cows as being the only wildlife in the galaxy. Specifically I was referring to civilised species, though I suppose the wildlife will shape up to be a bit more important in the end. Anyway, I'm drifting off topic here. It would be cool to travel back to old locations, but either way, I am sure Bioware will have many new places to explore and fall in love with. Can't wait!