There are quite a few games that I wouldnt mind seeing sequals of. Such as most of the Bitmap Bros games as has already been mentioned. But there are two games I would really love to see sequals for as they where and still are two of my favourite alltime games.
Moonstone - An updated version of this with more locations, more quests and many other enhancements would be awsome. If it turned out right I'm sure I would spend many hours playing it, just like I did with the original.
Pirates - This game is the one game that I find myself playing regularly even 15 years after having bought the original C64 version (I also bought it on Amiga and PC). Usually I get bored of games pretty bloody quickly but this is a game I will never tire of playing.