Former Valve writer Chet Faliszek recently responded to a user's comment who expressed the opinion that "making Half-Life 3 would be incredibly easy, the plot could go anywhere."
Faliszek doesn't think so, and personally, he's not interested in writing a script for a potential sequel. He agrees that for some authors, working on a sequel would be an easy task, but not for him:
When people ask me, "Oh, wouldn't you like to...?" my answer is no. I almost never want to touch something that already has established lore, a backstory. I don't even want to touch Left 4 Dead — I don't want to go back to old projects at all. I don't want people who remember all of this better than me to yell at me for changing some detail of 50-year-old lore or some rules.
Faliszek recalled his conversation with developers from Bungie. They were discussing a "big game" (not Marathon):
[...] all their games have one thing in common — a huge amount of lore. And that lore scares me. I don't know as much about my own life as there is in there, let alone their games. I don't want to write a script with all of that in mind.
Because of the "baggage" of established lore, Faliszek perceives working on a sequel as a real nightmare — there can always be someone who knows better and will criticize the author for a mistake:
Therefore, any "sequel" for me is a disaster and a nightmare that I don't want to be involved with. And I won't be. This is one of the nice advantages in my life — I can decide that, or it will be decided for me. I don't want to touch it with a ten-foot pole — and even if there's a gravity gun between me and that pole, I still won't touch it.
The video is available at the following link.