I'm very glad to hear you and your friends have enjoyed the game so much!
Our policy on fan content is that we encourage it, as long as some credit is made to the original game, and as long as you don't replicate the main rules in your own creations. SiWWTH has no Systems Reference Document at the moment, so we trust you to make the distinction between our work and your own with care and responsibility. Whatever you create should be in some way different from what we've made.
But yes, creating things like new settings, new monsters to play, and new abilities and mechanics is totally okay! We'd love to see it, so please do let us know if you make something!