The goal of this is to keep the loot hunt interesting without making everything overpowered, if you average 5-6% more damage per item slot, a full suit of gear can double your power. IMHO, that is worth playing for.
Any item can roll Primal, Ethereal, Mythic, or Epic. The best combination will include 1-3 Ethereals that boost damage + a combination of Mythic/Epic. In some cases Epic Gloves/Rings might be better than Mythic provided the affix rolls are above the baseline of a Primal.
Look at it this way for amulets:
10-20% Area Damage
As described, Mythic items will disappoint the most. So, I recommend guaranteeing the min range is 50% better than Ancient:
Mythic Amulet with Better Min
On a similar note, Raxx made a video talking about a cheap solution to avoid Primal disappointment. We salvage bad Primals into a dust reagent which are used to craft another primal. This Primal Disappointment concern also applies to Mythic, Epic, Ethereal.
If a greater percentage of items were actually useful, red beams would less often result in sad feelings – the same for teal, purple, pink, gold, silver or whatever other colored beams are associated with these new items types.
That said, the item revamps we’ve begged for should happen sooner than this.