I'd say an improved drop rate would be a good addition to hard mode.
Sort of like Guild Wars, where if you finished a dungeon you typically had one or two items drop from the chest (depended on dungeon) at the end, but if you did it on hard mode you'd have double the items drop.
But also have the increased drop rate applied to mobs you fight as well.
Also while in hard mode the level of everything you fight would be increased.
That would be a good incentive imo, though I'm not entirely sure if it would fit to FFXIV.