Apparently (apparently) Episode 3 of HL2 was going to have the Portal gun, and some kind of crossover (both games are part of the same cannon I think? There was some reference to the Half Life universe in Portal 2 from memory).
I always thought they wouldn't do a sequel until they had something revolutionary for the genre to showcase, like how the second game had the physics engine. Half Life Alyx fits that mould and looks unbelievable, but I can't get excited about VR let alone fork out the many many $$$ for all the gear to play it.