I have a feeling Bioware will not, nor anyone else, be doing a big-ass trilogy like ME1-3 for a LONG time. ME3 was 4 months late, and it was still rushed, which is why the original ending didn't make a whole lot of sense, and a lot of the little things (like the magic pistol that shows up every cutscene even when you didn't pack it) weren't to the same level of polish as previous games.
I personally expect a one-shot story, probably a prequel, though I'd personally find continuing the story further into the future a lot better. Just do what Deus Ex 2 did and mix elements from all of the endings. Just fit in Grunt, he was hilarious and I didn't get nearly enough of him in ME3.