Hi!
I wonder if it might be worth to create a prequel of FE 7. It could have a plot around the Black Fang former, Brendan Reed. It could explain the rise of the guild and why it was formed. Maybe a bad marquess/king somewhere (as you might recall such dudes were the first targets of the BF) - Maybe Laus again - threatening Lycia (again) by treason. Let Brendan Reed and his army which he found through the game battle him after a few chapters, let it be his first goal, something to aim for. He wants to create an organization for honor and justice, to fight the darkness of corruption.
The whole story would start 10 years before FE7.
Also, there are a lot of older characters in FE7 which could be used as well:
Lord Elbert - Although he would have already Elliwood as a son then. The hack could also be starring his wife as well.
Lord Pent - Still back when he was a pupil.
Karel? - When he was younger, like Guy? Maybe a plot which let's him envolve into a bastard as he is in FE7?
Hawkeye - Nabata desert....eh, something should come up into mind in time.
Marcus - Young, as a recruit cavalier.
wallace - A knight in service of house Caelin.
Legault - One of the first members of the black fang.
Renaud? - You could stare a scene were he murders Lucius' farther or shortly after or before that happens.
Maybe the story could be centered as well at Illia and Etruria a bit, whose were pretty much ignored in FE7.
Just a few thoughts, what do you might think about it?
I might help as well, but I have to say that I am totally new for rom-hacking, but shouldn't be such an issue with all the tools aviable. Though, I would hardly begin alone, because I think I need to people who keeps pushing me so that I don't loose the project, to keep it going.