A theory about Gullveig
Gullveig (also called Heiðr) was a witch that got burned on a bonfire by the Æsir 3 times and is belived to be the starting point of the Æsir-Vanir war, but why?
Yes Gullveig was a friend of the Vanir but why start a war becouse of it? Could it be that Gullveig is Njörðrs sister (the mother of Freyr and Freyja) and that he had enough of the Æsir who they probobly had some fights with in the past. Another thing i thought about is that both Gullveig nd Freyja is known to use seiðr magic so that could be something aswell.
I mean it kinda makes some sense even though some belive Nerthus or atleast an equvilant of her being Njörðr sister (and first wife) which could be true since Nerthus and Njörðr is a feminine and masculine variant of the same name similar to Freyr and Freyja. Freyr and Freyja is however sometimes replaced with Yngvi and Frigg so we don't know anything really.
Sidenote: I am no expert by any means and this was just something that came to mind so take it with a grain of salt
Some sources i used, take it as you wish and remember that this is just a theory of mine so if you have any other sources or anything feel free to share them
Gullveig: https://en.m.wikipedia.org/wiki/Gullveig
Æsir-Vanir war: https://en.m.wikipedia.org/wiki/%C3%86sir%E2%80%93Vanir_War
Njörðr: https://en.m.wikipedia.org/wiki/Nj%C3%B6r%C3%B0r
Njördr's sister-wife: https://en.m.wikipedia.org/wiki/Sister-wife_of_Nj%C3%B6r%C3%B0r
Yngvi: https://en.m.wikipedia.org/wiki/Yngvi
Freyr: https://en.m.wikipedia.org/wiki/Freyr
Frigg: https://en.m.wikipedia.org/wiki/Frigg