Throughout my life as a writer (about two or three years .-.) I've noticed some things... One of them being; writers don't write. Their stories' characters write.
Storyline comes to me over a long time. I don't just think and decide, "Ok, how about this person says this, goes over here, they fight this guy and get this thingy which..." Nuuu, but it doesn't really take a LOT of brainpower either...
Once you create the characters, they decide for themselves what to do and they carry on the story themselves. You often automatically know what needs to happen next.
The characters are what create the story AND the general feeling of it. For example; my manga Alpha/Omega is pretty lighthearted compared to other manga, but I recently (and very unintentionally) wrote in a dark, creepy part. This dark part of Alpha/Omega is caused and carried on by a really dark, creepy character.
On the other hand, the manga would be very serious if it weren't for a certain one of the main characters, who's very light-hearted and humorous. He changes the mood of the entire story just by being there.
I recently took an entire month to design a certain character's personality, and he ended up adding a lot to the story.
And so, my point is; take some time to make good, complex and interesting characters that fit the type of story you want. ^_^