I'd have trouble trying to think of a unique effect that's both balanced and attractive. I mean, I could go for something OP like Enables use of lvl 5+ Improve concentration, +6 dex and a slot and combos with spirit rings like clips and rosaries do. I prolly wouldn't fly as it does sound too hax, but People would definitely want it.
Or I could go for something a bit more derpy like a toxic ring that autocasts venom dust, envenom, and/or Venom splasher whenever someone gets hit. Because I've honestly never seen Venom SPlasher used. Ever.
The Oka Compendium: A glorified record of my alt-a-holic-ism. May or may not be up to date at any given point.