In my opinion the nerf to -enemy resist was a needed one and elemental spelldmg is at a good place now.
Not too op to oneshot everything whereas other dmg sources without this insane dmg multiplier struggled.
But the removal of -res hit flat elemental dmg as well.
Unlike elemental spelldmg, there is no other way to amplify elemental flat dmg on weapons (except direct melee hits...).
And unlike phys dmg (%EWD) there is no way besides -res to increase said dmg.
This change made most flat ele dmg on items (even lategame SU's and RW's) very underwhelming, except maybe the rather high flat rolls on rare and crafted weapons.
Since having more than one type of damage is so important, this nerf hit WDM chars without a conversion spell way too hard and made flat ele dmg way too weak.
And if you already have a conversion spell you would rather go for increased phys dmg than flat ele dmg anyway.
Even the sins ways dmg is rather low, considering you have to invest a lot of points.
My idea would be to boost flat ele dmg in some kind of way.
Either with a new stat like in PoE which boosts Weapon elemental dmg, or some kind of other multiplier. Or you could just simply buff the ele dmg values on items/skills for the lategame. (early game is in a good place)
TL;DR
Flat elemental damage is too weak and needs a buff.