The fictional Buffyverse established by TV series, Buffy and Angel, has had a strong cultural impact and a loyal fan base, and the creator Joss Whedon has been widely recognised as a talent within the entertainment industry. This has led to attempts to develop more on-screen ventures in the fictional 'Buffyverse'. However some of these projects remain undeveloped for various reasons: Sometimes vital cast members may be unavailable. Alternatively studios and networks which would provide capital for such projects might remain unconvinced that such projects are fiancially viable.
See Wikipedia for a summary of the undeveloped productions.
The Buffy wiki also has article on these undeveloped productions: