Yup, the expansions is currently named 'Aftermath' and is supposed to deal with the events after the destruction of the Black Citadel. And you're likely to play as Gordon Freeman after all:
I too would have liked Cpl. or Sgt. (not Col.!
) Shepard to make a comeback...why else did the G-man put him in suspension?
Having said that, there's mentioning of Shepard fighting in the 7-hour war iirc. In one of the newspaper articles on the wall somewhere. Meh.
According to Valve they want to get the maximum out of their engine in the XP, as they've become 'really, really good' at it by now. If this isn't some lame salespitch (which it can very well be
) it should be great. Having seen the stuff they did in HL2 one can only hope for the best if they're gonna stretch source to its limits.
read about I'll I'm yakking about here:
http://www.eurogamer.net/article.php?article_id=58579