In 2006, Whedon, working with TV & comics writer Jeph Loeb (now at Marvel Studios, unfortunately), pitched an animated pilot to Fox, which had discontinued its daytime children's programming slate 4 years earlier. Fox turned it down, and, ultimately, so did everyone else.
The following video was first leaked online in 2008. Most of the regulars returned, save for star Sarah Michelle Gellar, who, presumably, had decided to move on. Allyson Hannigan would later move on herself, to How I Met Your Mother.
Subsequently, any continuation of the previous series would be done in comic book form at Dark Horse, where it continues today. This had some promise. It's just too bad no one was willing to give it a shot.