It’s still not an idea.
The problem I run into most often with beginning writers or, heaven help us, people who try to give me their ideas, is that they are not ideas – not complete ideas. Say you approach me and say: “I have this idea for a story. There is a magical shop around the corner, and it disappears.” I’ll say, “Yes?” Why? Because that’s not an idea. It’s at best a seed.
In the same way – and this happens just as often – if you come to me and say “there is this girl and this boy, and they fight over the boat he’s taking,” it’s not an idea. Not even if you then add in a revelatory tone: “It’s the Titanic.” (Though that last one comes close, and I could probably start with that and write a pretty good romance or paranormal mystery short story.)
If you think J.K. Rowling’s idea for Harry Potter can be summed up with “there is a magical school” or “there is a boy and he lived,” then you will have to think again.
To make sure you have a story idea, you need to count the elements:
a) You must have a genre-appropriate spark. (That vampire-school idea is a genre-appropriate spark. That is, if I wrote it it would fit in either vamp lit or YA vamp lit. A spark for a mystery might be: “Man killed by shark. It’s murder.” It’s a spark because before you write, you have to figure out how to engineer a shark attack on command of the murderer.)
b) You must have a character, and there must be a reason why we – or at least you – give a hang about this character. This partly implies …
c) conflict, or a hint of danger or something the character desperately wants. (“There is Bob, and he’s happy and nothing happens” might be a nice thing for Bob, but it’s not a story idea.)
Note that the spark can be a setting or a twist on an old story.