I have an idea of how to make it more accessible and ready for players who don't want to chase down the uber bosses and grind out tons of gold. It also would avoid making the rings so strong that they get horded (and also make it near impossible to 'horde' them or dominate the bosses.
Sorry, this turned into a bit of a ramble.
To sum up - highly accessible bosses that small mid-tier or very small strong parties could kill. Make them fairly quick-spawn, but also to where players have to complete a path that doesn't make it too easy for simply sitting and camping; Rel-Thon's chest or Giant Chameleon style pathways for example. (party basically has to leave after accomplishing the goal)
Regular drops so it isn't a rare item, and then crafted with various levels of rings so people can customize their character based on race, class, and purpose at various levels.
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I thought perhaps making several mid-tier bosses that spawn frequently.
Then combine their drop(s) with rings for a reasonable balance mod-rings that customize characters.
In my opinion, reasonable balance is that it adds and reduces - so players can customize or optimize based on the character's race, class, and focus -
e.g. - Could grant extra armor but reduce strength, +char but -armor and/or wisdom, +wisdom but -dex.
I think making the items rather common would be ideal. That way it's not just big players with access, but everyone can get them and get enough to make multiple different stat mod rings. Crystal Rings are pretty easily accessible, but perhaps also allow it to be done with the various +4ac rings.
High customization from levels 25+ is what that would provide.
Perhaps multiple uses where people could use any kind of ring - Ring of Apollo, Gold Ring, etc. as well, and those with lower AC but also lesser debuffs to enable training of new characters.
Wisdom is the principle thing. Therefore, get wisdom, and in all your getting, get understanding.
-Proverbs 4:7