Yeah, a JK2 expansion would rock! I like Mysteries of the Sith, although apparently some people didn't. Of course, since it lacked the FMV of JK:DF2, it was closer to what JK2 is.
Of course, seeing as the game is almost a year after release, I'd say we won't see it. Most games that get expansions have them out within a year, an announcement is usually made within 6 months of release. Look at BF42's Road to Rome. The first Ghost Recon expansion was March '02 after a Nov '01 game release.
The last Lucasarts game that had an expansion was Galactic Battlegrounds with Clone Campaigns. That was a May '02 release for a Nov '01 game. Even the AvP2 expansion came out by June, less than a year after it came out.
If we see a JK2 expansion anyway (which I doubt), I think it won't be Raven as they're busy with Q4 and stuff.
Oh, let me add that I also would love a Hexen 3 or Heretic III. I was quite saddened that Heretic II got no addons other than that free MP map pack. Nice, but I loved the games for their solo play!
The Jedi Master