What themes have Kamen Rider not tried yet?
Well these have been tried
Knights (Ryuki/Dragon Knight)
Vampires (Kiva, Dark Kiva)
Onis (Den-O Sword Form and all of Hibiki)
Beetles as classic (You know em')
Techwarriors (G3, G4)
Time (DEN-O)
Forest Slashers (Gills)
I think these haven't been so far:
Mystic
Cyber
Agito?
I can come up with a few themes....
Frogs?
Fish?
Demons?
Space?
Cyberspace?
But speaking of frogs, I just found this impressive fan site about a fanfic Kamen Rider who is amphibian based. Very impressive work I would say. But the writer seemed to have abandoned it.
http://www.freewebs.com/kero2008/apps/photos/photo.jsp?photoID=15319082&prev=1