well... this is what happened to me while writing My Family's Boyfriend:

'Hmmm I should make a oneshot called My Family's Boyfriend! It will be cute and fluffy! And it could have Eddy Oh and take place in Japan!'

*gets to writing*

'Wait, what happened? This is all angsty... I got too many ideas! It's gotta have more!'

So write it and if it's a one shot it's meant to be. If it gets your creative juices flowing in with it. I'm not very good at one shots anyway XD I wrote one and it !