Should you write it? Well, that's not important. Let me tell you what's important.

So, you have this new great idea for a story set in your favorite franchise. You are very excited about it and think it's great.
But this is the point where you should ask yourself.
Should I write this?
Now, you may say that the whole point of fanfiction is to write whatever comes to your mind and that there are no limits.
And you would be correct.
So yes, you should write it. But the point then is not if you should or shouldn't.
The point is how.
You may have the idea, it may sound silly, it may be controversial, it may be something everybody else has done already (I'm looking at you thousands of Harley Quinn origin stories), and that is why you have to focus on how you're going to write it.
Because how you do it is what will set your story apart from the rest.
An original idea can only carry you so far.
A quirky or silly one can only capture attention up to a point.
That is why you have to pay attention and write your idea well, with good grammar, good presentation, good sentence structure, good characterization and attention to detail.
Because quality validates the idea no matter how strange, how controversial or how many times it's been done.
Or at least that is how it would ideally work. If all you want is attention, you can always write a popular pairing in your franchise of choice and end up with a ton of reviews regardless of quality, but that's something we can talk about some other time.