your sarcasm is noted

I assume you mean "antagonist", in which case it calls for an automatic boycott of the writer forever.
(edit: in RPGs. Miles Edgeworth is fine by me.)
(edit 2: obviously, this only applies to games whose original installments were good. If the original had terrible writing, and in the sequel I can turn the tables and wreak havoc, that's also fine.)

If you mean the same player-controlled character sides with another faction in the sequel, it's... questionable. I mean, if I wanted to side with them, it's not likely that I played the original game where I would have to fight them. Still, I would give five years of my life for a game in which I can conquer Azeroth in Illidan's name and make it stick.

Not subtle enough?

Only if it was done in silly family films. Can you imagine ET coming back for a sequel where he like fights the military and eats children? That would fecking rock.

Or maybe a darker sequel to Snow White where the 7 dwarves become possessed by an evil spirit in the woods and they proceed to gangbang Snow White for the rest of the movie.

as subtle as a sledgehammer my friend

I think it depends on the kind of RPG you are playing. If it is a game like Diablo where your actions doesnt matter and you have no control over the story, then it would totally work. You dont even need to write a good reason for it cos the players wouldnt care about it anyways.

However, if you are playing a game like Baldur's Gate "Or any bioware game" or Final Fantasy Games, then you need to be very careful. The players of the first game might go berserk on you for making their bellowed character go evil. Players of such games care alot about the story so you need to be very good at that plot twist. Even when I was writing a story, my main character went evil (in the chapter called "The Change" ) and audience revolted. So its a risk you have to calculate.

Wouldn't be surprised if a smut film would be made with that premise :P

I'm told i'm dense as lead, so i'm not sure if that's too much or too little... :P

Could be a bit like the Geneforge series, where your previous characters are mentioned in passing without details... although i don't know if they became the villain, only that they were still mentioned from time to time...

For RPGs, its complicated, but some other genres can do that nicely. although I saw it most in the same game, rather than in sequels.

* Adventure games, with their strong "story over character immersion" emphasis, can pull that, loike in Dreamfall, where you alternate playing an innocent bystander, a rebel leader, and the assassin that was sent to eliminate the rebellion.
And the "the character you played for the first half of the game was the bad guy all along" reveal is something I saw pulled succesfully in several different adv. games.

* Some story heavy strategy games are very adept at this, with the successive campaigns in a game. In some of them (Starcraft 1, Warcraft 3), you get not only to fight against the faction you played earlier, but also against your previous protagonists, some of them twisted into villains by the story.

Hey look it's the Sydney Swans guy

I returned to the thread to mention Donkey Kong Jr., but you beat me to it by hours :)

It kinda happened in Warcraft 3 : Frozen Throne with Illidan. In the final level you were fighting against him.

Terminator - if you watch in reverse order:)

Easy. Kain was bad, then became good, in his own way. Raziel was good, became bad for a short while, then good again. Vorador was good. The Sarafan general was evil. The guardians were, or to be precise, deteriorated into becoming bad. Moebius is very evil; the Cthulhu-like god is super evil. That random vampire woman was good, too. Any more questions?

No, that would be Cruella de Vil.

The Golden Sun games had a similar reversal. The second game follows characters who were on the team of the antagonists in the first game. You spend the second game trying to achieve what you spent the first game trying to prevent.