It has been argued on this forum that Soulbound Weapons are rather weak, at least when compared to major endgame weapon like Tidefall or Dragon-eyed Tallgrass. There is also the problem of Durgan Steel : Even a Fine weapon can be upgraded with it, but not Soulbound ones.
I suppose the Dev didn't want to add the possibility to Enchant them because their enchant level would be a mess as their properties are not fixed once for all...
My opinion is that such unique weapons deserve a bit of raw power. As they are already Superb, the most obvious choice would be a little Lash property. Currently, the fact they miss it means they do less regular damage than most topdog weapons and that's a pity...
I don't think it would make them overpowered (They would still miss Durgan Refinement. But this is not so bad as it saves Durgan Steel for other weapons.)
They could get the lash at Upgrade level 2 for example. Even without Durgan refinement, this would make them a bit more competitive.
By the way, I don't think it is too hard to add from Dev perspective.
Then, the funny thing would be to choose their lash effects. I think the following ones would fit really well :
- Stormcaller : This one is obvious -> Shocking Lash.
- St Ydwen Redeemer : Holy Weapons share a common trait with Demonic Ones : They like burning their ennemies down ! One of Redeemer's effect (Divine Mark) already does fire damage by the way. -> Fire Lash
- Greenstone Staff : Do you know what is green and hurts ? Either a Demonic bean or Acid ! -> Corrosive Lash
- Nightshroud : For the night is Cold and full of Terror -> Freeze Lash
- The Grey Sleeper : So there is no elemental lash remaining... However, there is already a weapon that has another unique Lash : Justice and its Crushing Lash ! Crushing Lash would fit really well a weapon that can already "throw rocks" -> Crushing Lash
IMHO, it fits so well that it sounds like destiny.