A few ways to cut down on inventory.

First off, enhances JA/Spell gear, for the most part, just make it so once you obtain the gear you can do something to permanently have the effects. Something like, Enhances Warcry for instance which improves the duration of a JA's effects would simply be made permanent. Things like Brutal Earring which does not really Enhance Double Attack, but rather gives you a % of Double Attack, would not count, because it does not really Enhance something you already have. This kind of change would cut down on pieces of gear literally used for a JA or Spell and nothing more.

Second, make more gear like Adoulin's gear, a lot of gear there is great for everything, like the Yao Helm from Tojil or the Delve legs for Light DDs, where these pieces are basically great for TP and WSing. Also, more pieces with DT and those kind of stats would help cut down as well. All of this can create some over powered characters in general, more the DT than the TP/WS part, but all in all it serves to cut down on extra gear we would use!

Thirdly, if its not gear, make it stack... Yes, I know, you fear that we will go harvesting a ton and will stack up tons of items to sell, really, not a threat. Making things like Logs and Ores not stack only makes it so more people drop them, putting less into the market, which to me is a bad thing. Also it forces us to keep items like this around for no reason, for instance I craft Gold Ore into Gold Ingots, but normally end up holding on to 2~3 of them till I do WoE or VW to get another few to finish a craft, during this wait I am eating up space I do not have. Its really a meaningless limitation, based on the fear of something so unlikely that I am probably sooner going to win a Mog Bonnanza than I would see people really spamming things for logs or ores for uber money when they are almost all so common and cheap.