Help with great weapon fighting. Designer required

Ello friendos, i'm in a west March server and a rules question came up regarding the great weapon fighting 

"When you roll damage for an attack you make with a Melee weapon that you are holding with two hands, you can treat any 1 or 2 on a damage die as a 3. The weapon must have the Two-Handed or Versatile property to gain this benefit."

Was it intended for it to work on the extra dmg of flame tongues, vicious weapons or even dragon wrath weapons? Or was it only intended for the weapon's base dmg die? One of the new designers from DND's team insight would be helpful. 

The designers of D&D don't really frequent these forums.

This Sage Advice answer clarifies that Great Weapon Fighting is meant to work with the weapon's damage roll, but not with any separate add-on damage effects. So, the question is whether the extra damage from weapons like Flame Tongue and Vicious Weapon is part of the weapon's damage roll. It certainly seems to me like it is, since these weapons say that the extra damage is "from the attack" and not from an external source, and that's the way I've always ruled it.

I don't think there's been any official ruling on it.

It is worth pointing out that Great Weapon Fighting is a pretty weak feat already, and if you don't allow it to be used on things like this, it's probably not worth bothering with at all.

Yeah, it's just like Savage Attacker: it works for the weapon's damage, but not from external sources. However, it can be tricky to determine what qualifies as an "external source" when there are many abilities, magic items, and spells that say "you weapon attacks deal extra damage", implying that these modify the weapon's damage directly rather than inflicting damage in parallel from a different source.

For example, the Helm of Brilliance lets you set your weapon ablaze and makes it deal an extra 1d6 fire damage. One could argue that the flame comes from an external source, but the fire damage comes from the flame itself, which is now part of the weapon.

Then you have Hex, which says "you deal an extra 1d6 Necrotic when you hit with an attack roll", which doesn't mention what the source of this extra damage is besides "you".

Spirit Shroud, on the other hand, explicitly says "any attack you make deals extra damage", making it clear that this extra damage comes from the triggering attack itself.

And of course, you have Smite spells, which all say "The target takes an extra [...] damage from the attack" which also makes it clear that the triggering attack is the source of the extra damage.