Add new set rings! I want a green ring to leave me in suspense!
No… fix bad ones before adding new ones
They’ll just adjust the drop chances so instead of 85-90% Cathan 10-15% Angelic, it’ll be closer to even. 
they are low level rings? How do you fix them without increasing their power and making them high level? Just add new ones and keep them how they are? Dense.
Who talked about making them higher level?
You easily make low level items as good for low level as they are for higher level
Using % is a way of doing this
You can also use *lvl mods
So you are saying change the stats on the existing two set rings? What are you even trying to say? The issue with set rings is the high drop rate and zero selection. You’re more likely to get a ring than any one specific item. It will also ALWAYS be magic meaning MF doesn’t help it outside rare/set/unique but high MF causes them to be set drops disproportionally and you only have 2 possible rings through the entirety of the game from Normal to Hell. Had I held on to every angelics/cathans ring I’ve found I’d have filled my 3 shared stashtabs completely, can’t say that about any other item in the game.
Your title speaks of new set items in general
But my point remains fix existing set rings before adding new ones
Cathan ring: 15-30% fire damage
Would be set fitting and work for low and high levels