Since we know how the game works when a shiny new idea comes do you ever find yourself uncertain because now that you're more aware of the market you have many considerations? Here's an example: You shouldn't write that contemporary romance with an edgy air because contemporary romance is in right now and who is say that it will still be 8 months later when you're finally ready to query? Or how about that boy MG. You hear they're looking for that now, but you know never to write to trends, but you have an idea that you love and truly want to write. Should you just call it quits before you even begin?
I believe that after a certain point of querying and receiving rejections on one manuscript and doing the same for another you learn a thing or two. Which yes, can be helpful to avoid the pitfalls that most new query peeps endure, but we also begin to look at our work in a more sales oriented way, which isn't a bad thing per se.
Still, I think we need to keep the fun in our writing for as long as we can. Yes, it's great to consider why you should and shouldn't invest your time in an idea that may not amount to much. BUT the best work, is the story we have fun writing, the one infused with our happy thoughts and yes, the occasional curdling scream (only occasional).
So next time that shiny idea comes to you--don't just knock it down. Give it a shot. Write down the first chapter or an outline, see if maybe there's more to it. See if it is exactly what you need to jump start those writing juices. Whether your goal is to publish it or not. Just go for it!