Overall I like most of the suggestion.
An "item belt" is an especially compelling design, as I think it would help me engage it items. By having to pick what I equip on each party members makes it more likely 1) items will be supporting their build and playstyle 2) I will actually use the things. As it is now, each character has access to such an overwhelming amount of items that it just becomes white noise to me.
Weighted shared inventory I thought was a very clever system in Pathfinder - a neat middle ground between tedium of managing individual inventory slots and irrelevance unlimited Stash of Pillars of Eternity. That said, I personally found it rather frustrating in Kingmaker, as game made me on couple occasions travel without companions, forcing me to drop and than reequip a lot of the inventory. However, as I haven't encountered such design in D:OS2/BG3 a shared stash might work rather well. I also think it would work just fine in Coop, as players could either share common stash, and have personal belts for items they want to keep private, or each player could be gited their own stash, and characters under their control would contribute to their carry limit.
That said, we are just waiting for 1.0 now, and BG3 got already an inventory UI redesign, and no design change was made. At this stage I don't hope for much or anything to change.
