In the AP, there are artifact cards and as an example i've linked to The Sickness card. It's a pretty ordinary item, all things considered, and isn't actually as powerful as you might expect of a Lv20 magic item in the hands of a low level character. But it's still useful! It grants a nice (if niche) passive resistance effect to diseases. Depending on your build, you might outgrow the passive bonuses, but that's not the end of the card's usefulness.
The card also has an enemy facing effect with a DC, something that is usually a death sentence for item's usability. Usually you'll have to retire items that do that a level or two after finding them (eg. Dread rune, yellow musk vial, etc.) because enemy saves are always rising, while the item's DCs never can. However, you might notice something about this item; It scales its DC with your class DC. It's just... usable forever.
I like that. More permanent items should be usable forever and hard to obsolete after you gain them. What's the point of questing for Excalibur if you have to retire it after a single adventure?
Yeah. The fact that items become obsolete is pretty annoying. I would like to have a system in which items can be upgraded as you level up, so you can keep the same things.
I'll admit, when our group first tried PWL, I had some pretty negative impressions of it - Medicine and Aid checks were suddenly a lot less reliable, and enemies we'd been critting and oneshotting were a lot more durable. Maybe it's worth a revisit at higher levels.
Maybe a variant that splits the difference, without making every item as bespoke as the Harrow cards? If there's a difference between an item's level and the level of the character using it, adjust any DCs by that amount. For example, a level 8 character with a level 6 Ring of the Ram would have enemies roll a DC 24 save (rather than the original of 22). That doesn't boost damage and the item would eventually fall off the curve, but might be useful for longer.
This would annihilate purchasing economy balancing and I have no idea how to correct that, but it could be useful for found or bonded items where buying and selling is restricted.