If you're having trouble writing anything new for this fanfic, what I'd suggest you do is to start working on different stories. Don't try to do the same concept as the one right here. In fact, try using a different source material (if you want to do Fire Emblem, try avoiding Fateswakening; FFN is already saturated with this content anyway).
You could also look to different source materials. It doesn't have to be a Fire Emblem game, or not even anime at all. Try branching out to different things so that you can find new perspectives of storytelling. With a story like this that you're writing, you could try watching "Rise of the Spartans", a Halo: Reach Machinima. It's pretty rough around the edges and it's also really long, but you could possibly get a few ideas out of the content and possibly apply it to your own writing in your own unique way.
Another thing is to write down any inspiration that you manage to get at any time of the day. If you get an idea out of random, write it down on your phone, a piece of paper, or even a napkin. The idea doesn't have to be in sequential order in the story, so you can write stuff down for later and adapt it if needed.
Nothing story writing related, but make sure you proofread so that you get rid of grammar mistakes throughout your work. Just a helpful habit that one should get into.
I'm sorry that I couldn't give you anything directly inspiration-wise, but I hope this helps. Good luck on your endeavors.
