Is that the ending isn't really the end. Think about this, when that reaper beam hits Shepard he should have been dead right then... end of story, that's it. However, we see him lying in a pool of blood with charred armor getting back up and shooting some more enemies before getting to the beam. Which takes us to part of the Citadel that we've never really seen before with piles of bodies all around. And now is where my theory comes up and that theory is that after getting hit by the reaper beam, Shepard is knocked unconscious and the part we play in the Citadel with the child and anderson and illusive man and all that is Shepard's conscience and his own internal struggle throughout the series coming to an end and he is fighting his own shame for not being able to save Earth. Think about it for a second, Anderson would be Shepards paragon half while the Illusive Man is his renegade half and the whole ending part with the mass relays being destroyed could be the Reapers messing with Shepards head and trying to make him believe that no matter what he chooses it wont make a difference or Shepards shame and uncertainty from dying so close to achieving victory over the Reapers. Also when you do save Anderson and shoot the Illusive man Shepard begins to crawl to the console that is supposed to activate the Catlayst while the whole time we heard Admiral Hackett calling for him to respond over the radio, but Shepard never does. Why? Because he is unconscious and that part where we hear Hackett is whats actually happening breaking through Shepards dream/indoctrination.Then after Shepard never responds he passes out/dies and then we see the child (perhaps Shepards interpretation of God, or the Gods, of the Mass Effect universe. Also maybe the part with the rest of the crew landing on an unknown planet that may or may not represent Eden/Heaven is Shepard hoping that whether his friends and squadmates are alive or dead they can now live on in paradise since the Reaper threat is over.
Now all of this might be hard to follow and I am jumping around all over the place in my theories about the ending but when you look at the teaser at the end where Shepard is lying in the rubble and then coughs/gasps for air it all makes you think that the theory I presented might be at least true. There is no way on Earth Shepard survived a direct blast from a reaper beam, then the blast blowing up the tank to destroy the reapers, then the citadel blowing up, then the fall to Earth, and finally being crushed by rubble. No way possible he survived that so he has to have dreamed the part with the Citadel at the end.