first of all, im sure you noticed (very sure) that very often in quests you can hear comments like "we dont do that opt, too low exp for it", well i did and i heard it often enough to get the feeling that it should be changed, so here is my brainwave about it:
all exp rewarded for completing optionals should be removed and replaced by % bonus to base quest exp, same way as all conquest/ransack etc.
this would achieve few things, first of all it would make optionals actualy have meaning in quests xp wise, second thing reward exp for completeing opt would scale on diffrent difficoulties much better as it would be based on base xp reward.
main idea is to make ANY optional have meaning in game, and i think it would be a good way to do it.