Shit I dunno. After 3 incoming episodes, it would seem they'd be enough to make up a sequel. Ignoring the weapons and areas, well if they add up to 12 more hours of gameplay, and they should, by the last episode, wrap things up HOPEFULLY in the way HL1 did, then thats a sequel.
As far as Gabe Newell's assumption that people want more core products and technology? Thats BS... Expansions are great because they aren't just in and of themselves, they are a part of the original game and they make the original game even better, and they use the original game to make it easy for devs to make good new content. Expansions simply add lots of potentially more fun experiences through more fighting, more enemies, more weapons, and more areas. Also I think a big part of HL1 was the multiplayer games they had, from CS to Team Fortress, maybe they should consider that for a while. But not till after they wrap the story up as completely as HL1 did (and do it quick!).