Well, we all experience this from time to time. My last time was with Conan. Now it is with Middle-earth, and particularly with Hobbit(no wonder why). Recently acquireing the One Ring game I got crazy about running a sandbox campaign set in the Wilderland. No not in my usual grim 'n' gritty sadistic style, but in a more light-hearted, fairy-tale style, the way the Hobbit was narrated. At this point I'm gathering lore for this endavour(wether it'll be ephemeral or not). The official forum for the One Ring RPG is very helpful, particularly few threads about woodmen and their settlements. I'm also digging through some of the older stuff like Decipher's dwarves' sourcebook and merp's mirkwood and misty mountains. If I don't lose interest in this I'll probably run a game or two via g+ since none of my real-world players show interest in the Middle-earth game.