I love these ideas. I’d like to put forth a theory of my own: each of the bondsmith spren are remnants or slivers of the Old Magic gods - stormfather as wind, the sibling as stone, and the nightwatcher as spren.
In WaT, we see ancient Roshar and the presence of the “storm” - a benevolent washing and restoration of the land, which, with the arrival of humans, takes on this idea of the violence of storms, becoming the stormfather as a spren.
In RoW the sibling speaks of properties of Ur, the mountain which forms the location of Urithiru, and there are references to the holiness of the location and its remoteness in relation to rest of civilization. Perhaps this mountain was the embodiment of the ancient god of stone? There’s also a mention of the word Ur translating to “original” in Dawnchant.
Then there’s the night watcher, who was transformed or preserved in some way by cultivation to remain a sort of spren of the spren, an idea unaffected by the thoughts and impressions of men.
