Or it could be a former N7 soldier who quit the military, or was forced somehow and ended up getting a ship and a few people to travel around galaxy doing whatever there is to do.
Or, going by this:
Thinking of the next Mass Effect game as Mass Effect 4 would imply a certain linearity, a straight evolution of the gameplay and story of the first three games.
That doesn't mean that events of the first three games and the choices you made won't get recognized, but they likely won't be what this new story will focus on. If you had three games centred around a group of key soldiers in the US army during World War I and then decided to make a game about another group of people during the Second World War, the games could have many points in common and feel true to one another.
You likely would have to recognize how the events of the first war influenced the ones of the second, but you would not necessarily think of it as a sequel. Again, the analogy is not great, but what I'm trying to say is that the ME universe is so rich that we are not limited to a single track when coming up with a new story.
It could involve exploring the universe in the distant future of ME. A point in time where Shepard's story is Legend and N7 has evolved to be associated with something other than military training. Perhaps the universe has reached a technological singularity, which is the hypothesis that accelerating progress in technologies will cause a runaway effect wherein artificial intelligence will exceed human intellectual capacity and control, thus radically changing civilization in an event called the singularity. Because the capabilities of such an intelligence may be impossible for a human to comprehend, the technological singularity is an occurrence beyond which events may become unpredictable, unfavorable, or even unfathomable
Two examples of these types of Stories would be Neal Asher's Polity stories and Iain M Banks Culture novels.
So beyond that point, Shepard's actions are noteworthy but the universe has changed so much due to the singularity that they are no longer relevant to how things stand.