There is no doubt that there will be an HGSS set. Based on the recent announcement of the release of the games, I expect an HGSS set to be after Advent of Arceus...
Here is out the next few sets be released:
PotF: JP: March 2009 U.S: Aug. 2009
AA: JP: July 2009 U.S (without movie cards) Nov. 2009
HGSS set: JP: Oct. 2009 U.S: (with AA movie cards): Feb. 2010
You heard it here first.
Anyway, I'd say there is a 99.999999% chance of Meganium, Feraligatr, Typhlosion, Ho-oh, and Lugia LV. X. That's at least...a little short of the standard now. I'd then say we'd get another one or two Level Xs on popular GS Pokes. I personally think Umbreon and Espeon LV. X, but it could be something else.
After the HGSS set, we could even get another semi-HGSS based set, like TRR was to FRLG. This set could include Team Rocket cards again, and some other Level Xs of popular GS Pokemon.
That's how predict the TCG will play out in the near future.