Metal Gear Solid 1 already had a script finished written by David Hayter (Voice of Solid Snake, Writer of X-Men 1 & 2), but various Movie Companies rejected it.
I REALLY Don't want to see a Tekken Movie, from early look's it isn't following the storyline of the game, so it will definetly bomb with the real fans.
Monkey Island would be a good film, but to really fit in all the jokes and make the story have any sense it would have to be around 2hr-2hr30min long.
The Discworld Trilogy, there were rumours last year about someone writing a script for the first one, and that the film will use the same Technique as Sin City but in colour to give off that imaginary world effect, but nothing is heard since

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The GTA's (from III) could be turned into movies quite easily, all they have to do is follow the storyline missions and Action parts.
Other Games that are in the works as films...
Dead Or Alive (the cast is done)
Driver (I think its the first one)