Sequel Idea
Let's face it, the idea of really making a sequel isn't a good one but if someone put a gun to someone's head and someone had to make a sequel, here's a basic idea that might not be too terrible:
Don's brother Malkie (Malcolm?) has finally put whatever differences he and Don had to rest, suspects Gal of foul play and so travels to Spain to get to the bottom of it. Malkie and Don could have been twins and therefore, Malkie could be played by Sir Ben, resulting in more great dialogue between Ben and Ray.
What say ye, team?
Come to think of it, you could do the same thing with Heat. De Niro said he had a brother (who could have also been a twin and therefore, played by De Niro again) who could recruit Val Kilmer and some other goons to get Pacino. Dreadful idea (De Niro and Pacino are too old now) but it'd make enough money if they got the whole cast and crew back together again.