Hmm, I got an idea for your inventory system.
I dunno if it would be any use, but still as this is an Idea, I will tell this.
You see, in real life, Items have weight.
How about that the number of items in the inventory would slow down the unit? /or maybe even specified to items types, so specific items would have specific weight.
Examble:
Hero running speed is 450 before picking up items.
Hero picks up items:
Item 000 [10kg]
Item 001 [10kg]
Item 002 [5kg]
Item 003 [5kg]
Item 004 [20kg]
The hero movement speed is now 400
Well, what do you think?
EDIT: Maybe an overweight also? and hero strength based carry.