So if I were in charge of this marketing campaign/reveal/whateveryoucallit, here's what this Bioware Announcement would be:
 
A post ME2 ending DLC for ME2.  Just that.  It wouldn't be like the Shadow Broker where you can start it whenever.  You'd have to finish the game first, and then this would continue the main story.  
 
That's what I would be announcing.  I don't think it's time for ME3 yet, tbh, and I think that theres more to develop in the story before the third chapter begins.  I would even go so far as to suggest following the Awakening convention (ie selling it as a disc/download based add-on).  From what one can see in the trailer/screenshot (there's not much), the story could be something involving your party having to land on a remote world to defend against an impending Reaper landing.  
 
To me that makes the most sense, because from what I could see of the end of ME2, it's going to take some time for the Reapers to get to the galaxy.  This opens a window for more 'bridging' DLC or an expansion, that tells the story of the first arrival.  That would set up the third chapter to be an all-out war with the Reapers, instead of this whole 'recruit/wait' story thus far.