Describe creatures that could have matched the Half-Life atmosphere (1 and 2 + Episodes) and could have been a good addition to the game’s universe in your opinion. What would they look like, what would they behave like? These could even be creatures that were in one game and not the other, or entirely new ones.
These are obviously not suggestions for Black Mesa!
For example, in HL2, I think it would have been interesting to have an enemy that would hide from you for a while, and try to attack you when you’re not expecting it. It could creep you out knowing that you’re not alone in the room, hearing it crawl around and knock things down from shelves, but you would have to hunt for it to kill it before it suddenly starts attacking you from behind. Basically it would not start attacking you right away, but rather lurk around like a cat for a while, and then suddenly jump at you. It could make some barely-audible noises, making you constantly paranoid about whether you heard something or if it was just your imagination, building up some suspense.
I thought of this because in every game, enemies always directly attack you: they see you, and they try to come closer and instantly attack you with whatever they have. In HL1, the houndeyes were the only things that came towards you then often ran away again. I heard that they were originally intended to be friendly creatures, which could explain their dog-like, non-aggressive, excited behaviour.