I would like to see voiddust made stackable, there really is no point for each 1 to take up space.
I am one of those people who do a LOT of voidwatch and use a lot of dust and while I have no problem with having to spend the gil to buy some off the AH or the IS/AN/CP having to buy some every time you go back to town seems a bit wasteful as far as time goes when you could just run up to the NPC like those using saved stones and trade each round.
There is no advantage other than having to make less stops at the AH/reward NPC in having 5 or 50 in your inventory as they dont give you extra's like the corundrums so I dont see this giving any issues to balance, just the sanity of your fellow party members so you dont make them wait while you make yet another stop for 5mins at the AH getting dusts.