Lyn truly should be in Brawl. So many good points:
-Female
-Has a bow
-Would be the only fast sword user
-Great face for Fire Emblem outside of Japan
-Great legs everywhere
-Really cool hair (style and color)
-A recent character
-Awesome, awesome music that begs to be brought up to Wii instrumentation at minimum
Edit: Seriously, out of FE7-9, Lyn's theme (and by extension Eliwood's, Hector's, and other variations of that music) is probably my favorite piece of music. Not for lack of competition, either: Determination, Together We Ride, and the Fire Emblem theme are also awesome. I just think I have to give the slight edge to Lyn's theme. (Though I may be influenced since I've been hearing Together We Ride and the FE theme for so long that maybe I've just grown more tired of them than I would be otherwise.)