Well, I always thought the most impressive example was the buffy fans. I know about five different virtual season 8 followed by a season 9 (and three of them even have even a season 10 currently in writing!), about three virtual season 6 for Angel (and a season 7), plus manu shows invented by the fans: a show about Willow, another about Spike, another about Faith (these three has at least 2 seasons each), one about Giles, and even one about Illyria! And since I don't follow all that, there is probably more that I don't know about! I would have given the links, except I didn't keep them all, and some are in french anyway, but that's darned impressive!
They even made false credits videos for the shows that doesn't really exist. The buffy fans really are something!