It's all in the way that you handle it

In terms of writing, there's no such thing as a bad idea.
Seriously. If you can marry Jane Austen and zombies, and give birth to a movie deal, and make international bestseller lists--twice!--with Twilight fan fiction, there is no possible way to screw up on the idea front.
Execution, on the other hand, will kill you every time.
(Yes, I intended the pun.  I get an o-pun-ing so rarely, I have to take them as they come.  Deal.)
It is, as Chuck Berry once said about sex, all in the way you handle it.  Experience and technique play a role.  (Don't they always... Geez, when did this turn into THAT kind of a blog?)
I mean, WRITING experience and technique play a role.  Seth Grahame-Smith of PP&Z had writing credits to spare before he penned his first mash-up.  E L James of 50 Shades of Gray had spent her time behind the keyboard, too--though in different areas.  But luck is a key factor, too.  For example, people have been writing fan fiction since forever.  And I don't just mean Kirk/Spock.  There's a distinct possibility that some of the more out there Arthurian stories, especially those with overheated bro-mances, were fan fictions written by educated ladies for the amusement of their friends. 
Why was James the first to go mainstream?  In part, because she lucked into the historical instant when a host of factors--the Internet, the popularity of her inspiration, popular awareness and access to fan fiction, etc.--her books were there when the public was looking for exactly that sort of story.
The flip is also true.  If she hadn't happened on that historical instant, she would've been dismissed by the arbiters of taste as having wasted a lot of time writing a BAD IDEA.
Piffle.
There are, however, perfectly good ideas whose sole purpose in life is to drive the poor writer into a state of gibbering incoherency--or in the case of those of us for whom gibbering is a way of life, greater blithering idiocy than usual.
For example, about two years ago a new fiction magazine (paying professional rates of five cents a word!) invited me to submit a story to one of their first issues.  We talked about the kind of story they wanted (contemporary urban fantasy featuring magicians and mundanes set in Washington, DC) and the specifications (under ten thousand words).  I did a proposal and a plot outline.  They loved the idea and sent me a contract.  Before it was even written!
Then I sat down to the keyboard.  By the time I reached my due date, the story (which had already been outlined, remember that) was seventeen thousand words and still growing.
I returned the contract with groveling apologies.  Fortunately, it was a "payment upon delivery" deal, so I didn't have to worry about the money.  But still, nobody wants a reputation as someone who fails to live up to their obligations.  In addition, I'd always been a reliable producer of nonfiction for the publication.  So the publishers and I are still friends, and the market remains open.
But without a deadline and the lure of ready cash at the other side, I stalled on the story.  For two. Whole. Years.
Then, about a month ago, I decided to open the file and see how bad a mess I'd made of things.  To my shock, the story was ready for the next word.  Over the next forty-eight hours, I blazed through the nearly three thousand words needed to complete it. 
I had a draft.  It was good.  But it was good at twenty thousand words, not my original market's ten thousand word limit.
Oh well, so now it's a novella.  There are markets for that.
It's all in the way that you handle it.
Happy writing!

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Jean Marie Ward