Thanks Bash, I was the original person who asked this question

If you're still up for sharing more info on this subject, will unique items (aka legendaries) have some unique stats that don't appear as normal affixes on other items? Similar to some in D2, where some uniques had a theme, for example azurewrath which focused heavily on elemental/non-physical damage and had an aura on-equip. I really liked that about D2 uniques, it set them apart from rares.

Yes, there are legendary specific affixes.