[Proteus]

"Space Opera" and "military sci-fi" seem way too broad to be successful.

...but I could see a GURPS Bug Hunt, with a book for player archetypes, a book for GMs looking to set up interesting tactical situations, a book with extra variations and different species of baddies, a book for integrating psychic powers, a book with extra equipment, etc.

Basically, I agree with the idea that successful lines are about obvious missions, not genres. Kill the monsters and take their stuff; break into protected locations, neutralize resistance, and secure the objective; find the monsters, kill them, and save the people. The mission has to be obvious, simple, and appealing. If GURPS Action isn't selling as well, then it may be because its focus is more vague than its companion lines.

GURPS Cyberpunk does seem like another obvious choice. It's got obvious archetypes; it's been the subject of successful, classic RPG lines that people may want to emulate; and it offers lots of room for expansion. Off the cuff, the line could include: a book with character archetypes, basic equipment, and rules for cinematic hacking, etc.; a GM's guide for campaign and adventure design; a catalog of additional weapons, equipment, and cyberware; a description of several corporations with adventure seeds; a "bestiary" of street gangs, corporate strike teams, and other NPC opponents; a book about integrating psi powers or magic, for those who like their cyberpunk that way; and so on.

I don't think of "Westerns" as a line so much as a one-off PDF like GURPS Supers, though it could certainly be helped by something like a GURPS Locations release detailing a small tone out of which to base campaigns, or a GURPS Thaumatology product describing Native American magic, etc.