I think the LB should emphasize SE's vision of Bard being a utility class. As lurkios stated above, playing off their song ability would be ideal. Since the usefulness has to be on par with the level of damage the melee LB can output (so people have an option), I suggest the LBs be songs with amazing effects.

Level 1: doubles MP/TP regen rate for X seconds
Level 2: instantly restores everyone's MP/TP to full
Level 3: instantly restores everyone's MP/TP and all party member's skills cost 0 MP/TP for X seconds

These aren't balanced obviously, but are merely suggestions to form a base