How can a continuation with Shepard not introduce new interesting companions and supporting characters? A new story is fresh because it's a new one, even if it's perhaps tied to older events.
How do you know everything has been told about Shepard btw?
It could introduce new characters, but it most likely wouldn't because they would most likely just bring the fan favourites back. If they didn't there would just be a huge outrage by the fans again whining about their favourite characters being sidlined (just like people complaining about ME2 companions being reduced to cameos).
And everything has been told about Shepard. Her/his story arc was the battle against the Reapers and that is finished. In two of three endings s/he is either dead or ascended to almost godhood. And even in the one ending where s/he is still alive there are practically no loose ends you could still expand into a whole game.
Of course you could throw Shepard into an entirely new adventure, but that would just look totally out of place and seem like an awful cash grab (Which ME4 wil be anyways, but they don't have to make it that obvious.) .