0 to play and 0 to equip is the only acceptable cost for the one Ring. From a lore perspective a random hobbit can use it so that is where the cost needs to be.
What other buffs does it need?
It needs to be templated to combo with Hobbits, which should all have an ability to negate negative status effects.
My only request is that the One Ring is so powerful and format breaking it starts on the restricted list.
Legendary Artifact - Equipment
Equipped creature gains unblockable and shroud.
At the end of each turn, if creature is not a zombie, put a +1/-1 counter on creature.
If creature dies while equipped and it is not a zombie then return as zombie and equip.
Whenever equipped creature deals damage to an opponent gain control of an artifact they control.
To elaborate on my testing of Bergi as a centerpiece more.
I don’t think I ever had a game where I was unhappy to see her and in multiples she gets even better, which is not typical of most cards.
The 3 mana mode is excellent value. She quickly pays for herself as she triggers off every spells, which is often better than cost reduction effects since she makes mana off 0 drops. The 3/3 body was big enough to guard my life total and pressure the opponent in different scenarios.
The 5 mana mode though is what is bordering on broken. This has to be by far the best “future sight” effect that I’ve ever played. If you have cards in your hand you can instantly dig deep into your deck as every card in your hand becomes 2 off the top effectively doubling your hand size. I’m not sure if it was correct, but I found myself wanting to pass after casting so that I could have all my mana untapped to cast everything that she finds. The interaction with Sensei’s top seems more like gravy than a necessity.
My final thought is that she may actually be better in a build that isn’t reliant on artifacts as her abilities work in any deck although I’m uncertain as to what that would look like exactly.