Living Artifacts - Artifacts that gain experience
How about if items equipped on a Sovereign or champion could gain experience? So when a champion gains experience, they could choose to put a percentage of their experience into one or more (must be) equipped powerful artifacts, or keep all the experience for themselves. This is a slightly reworked version of the concept of living artifacts from King's Bounty. Mages could craft (at a balanced cost) or start with (for those already with Path of the Mage) a special artifact that would increase spell effectiveness of one of their spell schools as they put more of their experience gained into it. Sovereigns or Champions could start with a living artifact, or find one or more. They should start reasonable and get better the more experience you put into it, but the experience would then not be used the boost the champion/sov themselves. Units have experience, why not have artifacts that gain experience and abilities the more you put into them? They could still have level, strength, dex or constitution requirements like other regular items.