Well, I've done a lot of fanfiction writing in the past in the Sonic the Hedgehog and Rockman.exe worlds.
Done a few things with original characters, also. 
I love reading/writing sci fi or fantasy. (Honestly though, it's often easier do do fantasy. Less techno-babble to try and make up.)

One of my favorite things to explore is elemental control, a la Avatar the last Airbender and such.