Modern Warfare 1, I wouldn't say it was amazing, but it was verging on greatness and a sequel should've been fantastic...instead it just stagnated.
Also, the Mass Effect trilogy, I would like to see the universe expanded upon in other games, but I just can't shake the feeling that it would go horribly wrong, especially with EA in charge. Also, turning Mass Effect into a franchise would ultimately lead to a canonical Shepard being defined, which would ruin part of the appeal of the main games.
And as much as I'm excited for it, I really think Star Wars: The Old Republic should've been saved until after the release of a KotOR III. If KotOR III is ever released, it would lose impact because we already know what ultimately happens.