The thing about sequels is simple: People want the same experience, only different.
So the idea is to take what was fun about the original Portal and ring a few changes on it (or as Portal's developers like to say, "iterate" on those key elements), and to do so in a way that extends the original story.
Accordingly, here's my rough outline for a vision of a Portal 2.
[NOTE: Some spoilers related to the story of Portal follow. Unless you never plan to play the game (in which case, why are you reading this?), if you haven't done so yet, you might want to do that before reading the rest of this post.]
- Player plays as a new test subject (not Chell).
- There are fewer (and slightly different) testing chambers. (Call it a separate section of the Enrichment Center.)
- Add some new mechanics to the original Portal's crate-on-button, energy-ball-in-cup, lifts, "unstationary scaffolds," and press-button-to-briefly-open-door mechanics. Examples: energy-ball-firing-turrets, movable energy-ball-receiving cups, and lifts that tip (to create Rube Goldberg-like actions with crates).
- GLaDOS is back, giving directions -- it turns out she had a backup unit.
- But while she's "still alive," she's not "feeling fantastic." (She exaggerated.)
- Her problem is that she only got one of her personality spheres automatically installed.
- It's the "nice" one, so she starts the game being helpful, if a bit confused.
- This time, GLaDOS is able to interact with some of the puzzles while the player's solving them.
- Perhaps GLaDOS has mobile minions that modify puzzle mechanics, or that have their own portal guns. (This could be a use for the flying robots of Half-Life 2.)
- What if GLaDOS could actively create her own portals inside the testing facility levels?
- At several points, the player finds additional personality spheres.
- Solving a sphere level transmits the new sphere to GLaDOS, who has a minion install it.
- Reintegrating the various spheres into GLaDOS is the primary story/gameplay motivator in Portal 2.
- Once a new sphere is installed, GLaDOS's personality changes, and she becomes... less helpful.
- "Less helpful" means that she progressively becomes a more active adversary for the player.
- Only GLaDOS can create a way for the player to escape the Enrichment Center.
- Only by restoring all her personality spheres will GLaDOS be capable of creating a "portal" to the outside.
- The endgame will be about "persuading" the reintegrated GLaDOS to let the player go.
- The endgame resolution will leave the door open to a (final?) Portal 3. People think they love trilogies.
- There will be cake. Somewhere.
- [Optional] Try to work small mini-songs by Jonathan Coulton (sung by Ellen McLain as GLaDOS) into the gameplay or story. That way it's part of the game, but not just another end-credits song. The original was so brilliant (and unexpected) that trying to copy it too closely could feel formulaic.
Same, but different.
[2008/04/20: On a happy note, at GDC 08 a few weeks ago Kim Swift acknowledged that a Portal 2 was in the works. Huzzah!]