Another series of Fawlty Towers would have been fantastic.
Another Spaced - altho Series 2 showed the writers were too busy chasing other opportunities or the idea was running out of legs (especially with the heavy - too heavy - reliance upon popular culture self-referential parody (example - Espisode one - the Matrix)
Another Futurama - why did they end a fantastic show? Ratings, I guess.
Another Filthy Rich and Catflap.