I practice my writing skills by writing fan fiction. It's a great medium: no deadlines or word goals, and you get free feedback. The only problem is that as fan fiction, there are certain things you need to include which are the design of someone else.
I've found that my writing is at its best when I'm writing about something purely I designed, be that a world, characters, or plot. But when the necessary things I didn't design come into play, my writing seems to wane. I've seen this happen several times, and I believe it's because I'm more interested in what I've built than in what someone else has built. I'm more interested in exploring my own characters, places, and stories, rather than someone else's.
Question: Assuming there's no way around it though: that is, I have to include those creations of others, how can I can bring my writing up par? How can I write the parts I don't create with the same interest and excitement as the parts which I do create?
Example: The fan fiction I write is based off of the lore of an RPG-style board game. The board game revolves around a great ongoing war. My most recent fan fiction centers on someone who (unwillingly) joins the war. The first part is about his backstory, and introduces scenes crucial to his character and overarching goals throughout the rest of the plot. The second part is about him joining the war. Before he can actually go on his first mission, he must first learn about the war, be trained for combat, and come to terms with serving in the army.
The first part is great, as it's all my creation. The second part is less so. The war and everything about it is a creation of the lore of the game. Once the character enters the war, the ensuing story feels... you might call it 'scripted'. The story, characters, and even places are all my own creation (or at least the development of them is my own), but they're built within and on a framework which isn't my own, and which I feel probably doesn't fit the story as well as if I had completely ignored the lore of the game.
I know the obvious question is: can't you just write your own story? If it has to be tied to the lore, can't you do so in such a small way that it isn't intrusive and leaves you to create your own tale? Yes, I can, and I have. This particular story has to be tied to the lore the way it is. Disconnecting it from the lore is not an option in this case.
So that's my question: since it has to include the lore the way it does, how can I write both parts of the story with equal interest/excitement?