My idea for Lyn's overall moveset concept would be that she's the yang to Marth's yin. No, she's not an Echo Fighter, lemme explain.
Marth uses a sword with power concentrated at the tip (to reference the rapiers many Lords start out with). Marth is about spacing.
Lyn, however, uses a katana. Katanas are wielded by unsheathing, striking very fast, then sheathing. Due to the inherent startup and endlag, Lyn would be about timing instead of spacing.
She'd be kind of like a mix of Marth and Meta Knight, where, like Marth, most of her attacks are simple, one-hit attacks, and like Meta Knight, her attacks are very quick and only last a measly 1 or 2 active frames. Those 1-2 active frames would hurt, though. Katana users have to hit as hard as they have to hit fast, after all. Speaking of fast, Lyn is very fast in her game, so in Smash I think she'd have mobility comparable to Sheik or Greninja, meaning she'd be the fastest overall Fire Emblem character.
Man, this is why I don't like talking about Lyn. I get excited at how she would play in Smash, then I make myself sad knowing that, at this rate, she's probably never getting in since her game is pretty old by now.