As much of a Fire Emblem fan I am and would prefer more Fire Emblem characters, Lucina just isn't worth the effort. She's too similar to Marth (much more similar to Roy and at least we can give Roy a unique P:M moveset), and it's much harder to make a clone in Crusade than in the official Smash Bros series. Someone would have to sprite Lucina's entire moveset rather than simply change up Marth's model.
When the devs add new characters, they should bring something unique to the game to be worth the effort. Lucina is not, and the only reason she should even be considered is because of her role in the official titles.