If i was to pick lines they say during LB, i would pick witty one liners.

DRG: "Fus-ro-da" (come on who could not see that one coming)
BRD: "Arrow to the knee, HA!" or you could have "Boom, Headshot" ( cause come on bards are full of it)
WHM: "Rise and fight on"
AST: "Silence!" Ok its not a silence but i imagine silencer from Dota 2...
PLD: "Come on and hit me"
BLM "Rain down fire and cleanse the world"

Cant really think of any more, would be cool if the chars did yell something but that is alot of work for someone so small.