So I was re-reading Today We Choose Faces (by Roger Zelazny, one of my favorite authors, probably best known for his Chronicles of Amber series and for Lord of Light). In it, there are vast city spaces-in-a-single-building, connected by slidewalks and existing on many worlds. And so the little story idea/sphinx looked up at me and said "Imagine a house of many doors."
Now, I already have a 2013 NaNo novel idea I've been chipping away at, but sphinxes are persistent and a little scary (don't let them see you're afraid). So I'm thinking of all the dark adventures that could occur in such a house (yes, Rowling's Hogwarts had shifting staircases; and Howl's Moving Castle had a door that could open in different cities; still, it isn't the base idea but what you do with it to make it your own). The faint glimmer of characters start shimmering in the corner shadows... Or are those the eyes of other sphinxes?
It can be tempting to follow a sphinx... but I will likely just jot down some notes and file the idea away for later...
What do you do when other story ideas come visiting?
I have a word document of nearly every story idea I came up from the last few years. Whenever I get a new idea (and especially during NaNoWriMo time), I it down along with the date. Then every so often I open it up and read through it to see if there's anything worth expanding upon.
Personally, I don't like to write novellas/novels using an idea that has been simmering in my head for less than a year. I need time to learn the characters, world build, plot, and most importantly, to learn if I really like the idea or if the idea is more like a one night stand.