Personally, I say don't make another Mass Effect game. We know the Reapers were successful in their previous cullings and making a sequel would adversely affect game play if you could import your ME3 save game. What will ME4 play like if Shepard chose to destroy (or coexist with) the Reapers? Do you load the save game and then the credits just roll as there's no point since the timeless galactic threat was defeated. Shepard could also end up becoming the bad guy if he chose to control the Reapers. Choosing nothing and allowing the cullings to continue is the only viable story line for a new game.
Furthermore, if you couldn't import your save game to the new ME from ME3, I think there'd be a bigger fan outcry than the one for ME3s endings.
As a side note, I'm actually interested in seeing a prequel featuring the ancient race (30+ million years prior to Shepards time) that disabled a Reaper using a massive mass accelerator (from ME2).