I would do one of three things;
1. Make a prequel detailing some important event that would eventually lead to the events of the original.
2. Make a game set during the events of the original, from the point of view of a completely different character(Like the Blue Shift and Opposing Force expansions to Half-Life).
3. Make a completely different game, with a story that is, for the most part, unrelated the the original, but set in the same universe.
Whatever I chose to do, I would try to add new gameplay elements, new characters, new settings, new puzzles, new weapons, and new goals, as well as having a mood that is distinct from the original.
YuheJi said:
I would probably make a prequel, or do the story from another character's perspective (which would be more like an expansion pack than a sequel). That was what I loved about Blue Shift and Opposing Force.
I had already gotten through about 90% of my post when I scrolled up and saw yours. I just thought that was kinda freaky XD