I've been thinking about the Barbarian transmute.
It's supposed to be an amulet, so it has to be better than
tokendb.com/token/medallion-of-valhalla/
which is a tough bar to beat.
I am thinking it should interact with Rage. Relic: 1 extra use. Legendary: unlimited uses. That does not seem unreasonable to me.
Another option I would like is boost the Damage Reduction ability. Relic: +3 DR. Legendary: +5 DR. That may not be as exciting but it would be pretty powerful considering it affects all sources of damage except Push and Eldritch.
Or how about: Relic: 1/game go into an Inspiring Rage for +3 dmg for the party. Legendary: 1/game go into a Greater Inspiring Rage for +5 dmg for the party. This would be separate from Rage / Greater Rage which would still only affect yourself. So you would still use regular Rage in one combat and then Inspiring Rage in a different one. I like the teamwork aspect of TD and Barbarian and Fighter are the only classes that can't help the party/teammates in some way except by doing damage. Even Dwarf Fighter has Taunt, Monk has stun, etc etc.
Just throwing some thoughts out there. I would love to hear other ideas. I wouldn't expect any of these to be used verbatim. Based on the class legendaries we have seen so far, they have all been some combination of 2 or 3 effects into one token. So I would certainly hope for the same for the Barbarian!
Classes Played: Barbarian (65 times), Monk (56), Ranger (33), Rogue (25), Cleric (21), Fighter (13), Druid (12), Paladin (11), Dwarf Fighter (10), Bard (7), Elf Wizard (2), Wizard (2)