One of my favorite games and shooters, Half-Life 2 totally expanded on the first game with bigger environments, new sets of enemies (some of which are totally terrifying) and real-time cutscenes.

A very balanced and fun single player adventure, back when your character's health bar wouldn't refill automatically after a small amount of time (which was not all that long ago).

They totally need to make Episode 3 (as promised for long ago) or Half-Life 3 AND keep that style of gameplay, while introducing new ideas of course.