MAKE potions, echo drops, Holy water stack to 99, distilled water all healing items please. With that change make tanks to be chargeable with stacks of 99. Other Final Fantasy games have them stack higher. It's not game breaking to make a market for these it gives players the ability to spend money for healing. It's very important that the tanks start with 0 charges when crafted. This way an npc can be setup to fill them like nin tools are stacked. Trade tank + stacks, NPC ask for money and if the player accepts the tank gets filled.
It'd open up the market for alchemist
Make, ores, plants and logs stack to 12 then set a fatigue limit to around 200 items per day for mining, harvesting and lumbering. (With the exception of key item quest)
Make a furniture item that stores all elemental crystals/clusters. You can trade crystals/clusters to it or remove them as crystals/clusters by using it. These items shouldn't take up storage space.
That's all for now. Not like any of this will every be done like this because the devs think it'll break the game. lol Really, if you want to break the game just keep these items like they are now as a waste of space.
BTW: Powder although I like your suggestion to increase the duration of jigs it isn't an ITEM change. This topic isn't about Job abilities or spells it's about ITEMS.