While sitting on the microbus (only 10p to ride, but no oyster card system; I'm not sure if this means its better than London or not...) I started thinking about a far-off, fantasy world (as I often do)...but suddenly, it came more and more 'alive' inside my mind:

Characters, islands, history, all part of this 'made up universe'

Specific scenes developed, and slowly an overall plot formed.

Later on in the day, I was walking around town (lost), and I was thinking more and more about this...In my opinion it has the potential for a 'proper' story, like a novel.

So should I start to write it? Or is this a big waste of time, which I will give up on in a few weeks and then find the file on my computer and be hopelessly embarrassed of?

It would be for children/teenagers, but would deal with quite sophisticated themes/emotions/ideas. Not really like Seraph's 'Guardian Angel' at all, think more Tales of Earthsea, Discworld or His Dark Materials.

Is this a good idea?