This has got far too many pages to scroll through considering I'm only on my laptop to write an essay, so sorry for any repeats...
Invader Zim, Undergrads, Clone High and Downtown all had potential for more, Jhonen Vasquez was planning a movie to give an end to Zim but like everything he takes a break from, he never went back to it.
Spaced is my all-time favourite programme and a lot of the fans constantly beg for a third series, but I think it ended perfectly where it did, too many good sitcoms get flogged to death and get progressively worse, just look at Only Fools And Horses or Two Pints Of Lager And A Packet Of Crisps.