Combination items.
One of the best parts of Dota is the way low items are combined to make powerful ones and i think this should be added to Demigod. Im sure Stardock is sick of hearing about wanting Demigod to be more like Dota, but its not like Dota invented this type of thing. There should be another tab in the artifact shop for items that require lower items to create, or make some of the current artifacts require other similar low level items. This is a great way to free up inventory space.