I've got a magical MacGuffin. One that my character stumbles on -- rather like Bilbo stumbles on the Ring, in essence, though I never came up with a version where the details resembled each other. The character always started his adventures by finding it. I noodle around with the notion of its being a ring. I could always call it a tribute to The Lord of the Rings, and then I always thought of it having runes on it. Visible ones, and not the fine script, only seldom seen, on the One Ring, which would differentiate.
But then, the MacGuffin had to do something. It had to be Bad News, or the character would not have to deal with it. Too obviously ripping off the One Ring would push it well past the borders of a tribute. Besides, I didn't want it to be immediate trouble. The whole notion I was noodling with is trying to fix a problem before it gets bad, and when no one thinks it urgent.
And then, finally, I read a throw-away comment in an different work about sea serpents pulling boats. A character came wandering out with her boat and her sea serpent, wild cat, and bird, and explaining that she, in an old picture I had seen long again, was the main character, and her boat drew on her sea serpent, and the outline can begin. But I still didn't know what she was lugging about.
Well, I was sitting around at Arisia in a panel having nothing to do with MacGuffins, and for some reason pondering what the runes could say. And the story of its origin came together: the runes had been altered. And then what they said on either side of the alteration came clear, and my reaction was, this is not a power that logically goes with a ring. Magical items should be metaphorically suitable for their powers. To which the response was, it's not a ring, it's a horn.
So, I have my MacGuffin. Onward. All I have to do is figure out where the heroine can get some philosopher's stone, or devise some other way for her to work around the magic. . . .
I kept outlines. I also keep sheets where I just jot down ideas that smell like they go together. There are times when I really need them.