There are a few days when I wish I could write a fanfic that stayed within the limits of LBT, but truth be told, I cannot do this. I have tried before to write G and failed miserably, and only sucessfully made a few PG rated fanfics.
Do you ever wonder why you have trouble writing G- or even PG-rated fanfics? I agree that this does indeed place limits on the freedom available, but this goes back to something my music teacher taught me in high school:
I tended to go overboard with the variety of themes in my music compositions in high school. My music teacher talked to me about this and taught me the importance of theme, and staying within the boundaries of my own work.
I believe this applies to any form of art, even a fanfic. The atmosphere of the original piece can be related to more directly if similar themes are used, either in a similar style, or in variation. To me, the LBT universe has certain 'themes' that should probably be adhered to, in order to preserve the LBT feeling of the fanfic. Drifting too far from the basic theme results in a situation simiar to what my music teacher was talking to me about; something along the lines of:
"Goldilocks is wandering through the mountains and runs into 3 bears who get chased away by the three little pigs who get shot by the 7 dwarfs who are then killed by malificent throwing apples at them."
It gets so out-of-context that the original idea of Goldilocks is lost. This holds true for fanfics as well. If the LBT atmosphere is desired, the story must stay withint the 'theme' boundaries. This includes keeping it more-or-less family-oriented, as that is a major part of the LBT universe.
But to be honest, another reason why I would have trouble writing a "true" LBT story is because staying within the boundaries would be too difficult for me.
Perhaps you simply need more practice. Writing doesn't come easy to me, but if you honestly wanted to learn to write a G or even PG-rated LBT fanfic, I'm sure you could with some effort. Just give it an honest shot. If you get stuck, think about Why you get stuck. There must be some reason that keeping it G-rated is so hard for you. Consider this and solve the problem at the source.
Just a thought. B)