Omega. I mean come on, we don't get to see the Asari Godmother VS. Cerberus?
As for the theory that it's all just a dream, it's a rather specific dream to have. My problem with this, however, is that I assumed he was still on the citadel when his body convulsed at the end. Rubble is rubble, after all. I would be okay with this ending being real minus the stupid Normandy crashing on a distant planet, hope that's just bullshit since it makes no sense at all. I just want my decisions to have mattered for something and I have gotten the feeling that many of my decisions didn't matter at all. Killed Wrex? Pretty sure the other dude would have been there. Killed the Racchni? Don't worry, you didn't kill all of them. Brain-washed the Geth heretics? Oh...well, the Reapers brain-washed them back, and no matter what you did the Quarians are going to start a fucking war while the Reapers destroy the Galaxy...