Duke Nukem...
Ok no in all seriousness Nexus Jupiter Incident, as the developers were so far ahead technically with the game and it was an amazing flight simulation action game in space.
The story also has left room for expansion and to make matters worse the tech demo they released for Nexus 2 which they were unable to find a publisher for to fund the game due to Nexus 1 not doing so well (probably due to lack of advertising and due to too many bugs)
Can't really think of any others most of the awesome games I can think of have already had new ones in the franchises or still has new games on their way. I would like to see a KOTOR 3 some time sooner than it will be coming out though, as BioWare is busy with SWTOR and after which we will have to see if they start work on KOTOR 3 or not...