They really need to make a sequel to ME3 to wrap up the current storyline and leave it in a way that allows for prequels and sequels to be developed properly and with a "clean slate", even if those sequels or/and prequels aren't based on Shepherd at all. If you look at how ME3 ended, it was basically "Whatever you did over 3 games didn't really matter cause you're screwed either way".
They need a rosey garden ending where most of the universes problems are near enough fixed, otherwise it's going to be hard to develop a new game that is built on a universe that is doomed regardless of whatever you going to be doing in a new game. It's the same for a prequel, whatever you do ultimately doesn't matter as Mass Effect 3 so bluntly and annoyingly pointed out.
So yeah, either make an expansion to Mass Effect 3 to wrap it up properly or make a sequel to Mass Effect 3 involving the Shepherd and crew characters and wrapping that up properly before attempting to move on to a new story arch.