It's a balancing act, and BioWare does not want to fall.
We know the Mass Effect trilogy with protagonist Commander Shepard is complete, but we also know that the developer is currently working on a new franchise story arc.
It won't be called "Mass Effect 4" but the bottom line is that a new ME installment is definitely on the way. And in posting on Twitter (via VG24/7 ), BioWare Montreal studio director Yanick Roy said simply-
"…we'll be sure to be friendly both to old fans and to newcomers."
This was in response to the question as to whether or not the Genesis system would return in the new game. In regards to retaining the open-ended nature of the series, Roy said "there is no reason to believe otherwise at this point." Well, of course not. Linearity is dying fast; developers are going in the other direction.
We still don't know a lot about the new ME, but we know it'll use the new Frostbite 3 engine, and we know the team has "borrowed" some concepts from the Dragon Age: Inquisition team to speed things up. Personally, I want Inquisition more, but that's only 'cuz I prefer the fantasy setting. Don't care much about sci-fi.