Here's the thing. For what reasons would the game really be necessary? Unless they put a lot of effort into it, the game would only appeal to people who've already played Fire Emblem: Blazing Swords. And the people who have played it should already understand that - before Nergal used it as a host for his influence to spread - the Black Fang originally harbored noble intentions. Whatever happened to the Black Fang after Blazing Swords is beyond me, but there should be an adequate understanding of the Fang's history already.
I suppose their activities and achievements before Nergal would be of interest, but Intelligent Systems would have to do a bang-up job of making it a genuine story of its own.