Another way is to add more ways to get higher tier items :
- arena fights
- duel a lord for an item
- steal items e.g. when sneaking into a town
- items as quest rewards
- items as presents for marriages
- items as presents when joining a faction as a lord
- bartering for items with NPC lords
-items as payment being part of diplomacy, e.g. as part of a peace deal between factions
- paying a craftsman to make an item
- adapt looting scheme to include (lower quality but high tier) items from the player side as well as the opponent's side
- make certain unique items of artifact level quality that are available under specific conditions: e.g. the equipment of the ruler of a faction becomes available when the player is in control of that factions capital. Take a look at how the sword of Khaine is implemented in Warhammer total war 2 - it is an exrtemely powerful item that changes hands when the wielder is defeated in battle. May even make for a casus belli