(If he's in there and still alive, I will seriously lose my marbles.)
Anyway, given this seems to be a hearkening back to classical Final Fantasy titles where light = good, so more light = more good, I'd say killing all the Ascians and preventing Zodiark's revival is probably our ultimate goal. Any number of theories can be thrown around, but until Elidibus feels like sharing with the class, that's all we have...