By the way I found another thing that hints on a future DLC featuring Harbringer and the Collectors:
Harbinger and the Collectors are effectively an ace in the hole for Bioware. I'm expecting they'll save their return in the single player campaign for something truly epic, maybe even their last DLC.
Anything in single player that involves Harbinger and/or the Collectors is guaranteed to sell like hot cakes. Combine that with a final, shock twist in the tale that leads us into ME4 = welcome to Hypesville.
Plus, I don't believe for one second that Bioware went through the trouble of redesigning, reanimating and rebalancing the entire Collector faction for a free multiplayer DLC only; that's a hell of a lot of hard work, and re-using the assets in a single player DLC is a way to make it pay off.
FInally - look at the end of the Retaliation trailer, where Harbinger drops down onto Firebase Dagger. That is not an animated cut scene. That is footage from the game engine. An in-game Harbinger model, on an in-game map.