I believe the breath scene and the one more story is there to bring Shepard back for another game, not saying it will happen, but it leaves the door open for that possibility
I always thought that the breath scene was just added so that in future games they could reference some people believing Shepard is alive. With no intention of bringing Shepard back but people in the MEU believing that Shepard survived. My reasoning is slightly tenuous but there are significant messianic aspects to the Shepard story.
1) The name Shepard. A shepherd is how Christ is described, I don't think its an accident that the names are phonetically identical
2) Death and Resurection
3) Control ending gives us an Ascension mythos
4) The Companions are like Apostles following Shepard and obeying him even when they are not military
5) Committing seeming miracles (at least on the surface) like curing the genophage (even if Shepard didn't personally do so), Solving the Geth/Quarian conflict, Saving earth, Entering the omega relay and returning when everyone else before that had died.
6) being prophetic about the reaper invasion as if Shepard had diving knowledge
7) Association with an Enkindler. Not just an association but an Enkindler was subservient to Shepard.
There are symbolic aspects that are possible to the story of Shepard that are very messianic. (And yes not all players will do all these things) the point is they are part of the story's Zeitgeist. I can see in a post reaper war galaxy the cult of Shepard arising where Shepard is viewed as more than mortal. The hanar have the enkindlers, so having a group of people believing that Shepard is part divine seems very much a possibility. And you can't kill a god so there has to be this idea that Shepard could have survived and the breath scene creates that "possibility" within the community. Are the Shepard sightings you hear about in the next game, real? Did Shepard really survive? If there isn't that breath scene then it has no impact on players as they "know" Shepard died when the crucible blew up shooting out the energy beam the solved the reaper problem. However the breath scene means it could be true, it will mean some people will adamantly believe it and other that wont without definitive proof.
Its this is all speculation, I don't think you will ever see Shepard in another ME game set post reaper war. I could be wrong but I am pretty confident this will hold true.