Your comments

Can't see an edit button so I'll add what I completely forgot but seem essential, it's probably related to Duffer request, but I already said it, I don't understand what he mean and I think it needs to be described as generic as possible 

  • Item alternative based on a main item
    • Example is that I could have a Main inventory item (let's call it Basic Rifle) which has a set of actions predefined (equip something, launch this and that action, set those variables).
      But I would like also 15 other kind of rifles that derive from it in the way that maybe the model change (override the equip action) and the stats (as described in #5). the actions/variables etc could be the same as the main item but the stats (what it does) would be different
    • UI wise it would also let us organize things. Maybe we could fold the main items and click on a button to expand the list and keep things organized 
      • Create main item 1 Basic Rifle
        Create sub Item 1.1 Awesome Rifle
      • Create main item 2 Basic laser sword
      • Create sub Item 2.2 Unicorn laser sword

Also as for the Module itself. I personally would like to suggest you to keep the current inventory system and rename it "Basic Inventory module" and make that one a new module (advanced inventory sounds right). First of all because this will cost you dev time and that is money. Second of all, because maybe some people wants just the basic inventory as it is. If features are clearly explained in both assets (and that on both you warn people that both exists - it's fine)

Price wise maybe a reduction for Basic inventory module owners (if you think it's fair) and a base price obviously higher than that basic module.

That part is of course entirely up to you but I refuse to not participate financially to that module dev.