I like SciFi and Fantasy I think 20 minutes is good at least a length of episode
but I like humor more
Maybe adapting something in public domain that isn't already an X minus 1 episode.
Horror works well with audio oration or audio drama but seems it has been done almost too much especially around Halloween
You might find Audio Drama Talk website and its forums useful or interesting
http://www.audiodramatalk.com/
some of my favorite audio dramas are
Bell's in The Batfry
http://thebatfry.libsyn.com/
Tekdiff
http://www.cayenne.libsyn.com/
Decoder Ring Theatre
http://www.decoderringtheatre.com/shows/
the Radio Adventures of Doctor Floyd
http://www.doctorfloyd.com/
Otis Twelve
http://feeds.feedburner.com/OtisTwelve