Currently there is 4 slots for battle items. There is plenty of space to expand that to say 6 slots or even 8.
The expansion slots above 4 can be made available by forging items together, say several backpacks, some iron and another rarer ingredient.
Those additional slots would only be available on the Team # it was built for.
Artifacts - non-consumable battle items, would be special items that could be used once every wave or entire battle.
Some special artifacts would be only available/usable to a team if it includes X heros of a certain class or colour.
Some items can have static effects:
king arthurs sword - gives stat bonuses to your king arthur
pirate flag - all pirates of corellia heroes get defense bonus
spellbook of destruction - all wizards and sorcerers on your team get attack bonus
rod of healing - healing bonus on your team
cursed skull of berserking - all barbarians and fighters on all teams get attack bonus and defense penalty
heart of the forest - all green heroes and enemies get health bonus