[In this opinion piece, GameSetWatch creator and Gamasutra/Game Developer publisher Simon Carless muses about what it would be like in a world where the bad guys ruled the world of games - and, heck, who are the bad guys, anyhow?]

"This was a triumph - I'm making a note here: huge success."

As everyone knows, the one truly memorable character in Portal was GlaDOS, the deranged AI in residence at Aperture Science. As a first-person title with a relatively anonymous lead character, GlaDOS is the character I keep returning to - because she's funny, and unexpected, and beguiling.

So it got me thinking. Could a game work starring the homicidal computer as the lead character? You could argue that this would make approximately as much sense as a 2001 sequel starring HAL. Yet games work in a very different way narratively to films, so that flippant comparison may be a little, well, flippant.

As a result, I've been starting to question the meaning of the antagonist in games. Could there ever be a game that makes you play the bad guy in an existing franchise, but changes the gameplay accordingly alongside the perspective shift?

Luckily, we've asked a similar question before, and the answer tends to be that it's the mascot-style games in which this happens. Some of the earliest examples are the most interesting - after all, Donkey Kong is the antagonist in the original arcade game named after him, before switching to protagonist in Rare's SNES platformer series.

While we're discussing Nintendo, Wario is probably the best example of video game antagonist turned protagonist - and it's particularly fun because his games reflect his dastardly nature. But he is essentially a Bizarro Mario, with fairly conventional mechanics to match. Oh, and Dr. Robotnik's Mean Bean Machine is deliciously tangential, but still... tangential.

There's also Triumph Studios' Overlord, of course, an antagonist view of a world that there's never been a protagonist-starring game in. But it's replete with the kind of humor you'd hope for, and is squarely in the tradition of earlier games like Dungeon Keeper which rain derision down on the stylized hero.

Appropriately, one of the most interesting protagonist/antagonist flips, albeit not so direct as the above examples, is the Gearbox-developed Half-Life: Opposing Force, in which the player is one of the enemies from Valve's original Half-Life. There are no major gameplay changes, but it's an altogether less cartoon-y way to switch perspectives.

So how about it, Valve? The entire 'god game' genre allows those currently ensconced in basements to believe that they are taking over the world. Why don't you go all Evil Genius for a spinoff?

In this slightly tortured game, GlaDOS can be designing nigh impossible traps for 'test subjects' to traverse. Throw some Tower Defense-style enemy and obstacle placement in there, and you've got a game. You can even pick varieties of cake for victors to hypothetically snack upon.

Still, get anyone to evaluate the chances of this game being successful, and I think they'd be pretty skeptical. Most passively acted games, like the Deception series, just aren't that popular because, y'know, most people don't like being passive.

Thus, there's two approaches. There's the mascot-style absolutism. But today's forward-thinking game is dealing with this issue in a completely different way. Why get the player to empathize with the good guy, and then completely switch his allegiance to the bad guy for the sequel - a bit of a stretch - when you can be all these things in one game?

In the modern era, BioWare's Knights Of The Old Republic was one of the first titles to do this, with some obvious Jedi/Sith themes to riff off. And at least one of the top games of this holiday season, Lionhead's Fable II, builds its entire hook around the protagonist have the choice of whether to be good or bad. It's malleability taken to extremes.

One of my favorite movies, Jake Kasdan's Zero Effect, has Ben Stiller's character Steve Arlo explaining to the slightly overwrought, self-deluded 'world's greatest private detective' Daryl Zero (Bill Pullman): "There are no good guys. There are no bad guys. It's just a bunch of... guys."

In today's post-black & white society, this maxim equally applies. Which is probably why the answer to this column's question is subtler than you might think. After all, what games like Fallout 3 and Fable II show above all is that, in this nuanced world, it's not really about the barrel-chested hero and the moustache-twiddling villain. It's just a bunch of... guys.