Mmm I know, but I am a fan of strict cathegories rather than loose ones.
How about some sort of code? Something like this:
A=alien life forms included P=parallel world S=Sciense involved M=world set in time around middle ages M=magic involved
Then you could have a
MMP-novel for example.
Anyway, that might take it too far, but if you don't use a system like this, you'll always have trouble with definitions, same goes for music genres for example.