If we focus on the obvious HP Lovecraft connection for a moment, they could be related to the Deep Ones under Hackdirt. "Old/Elder Gods" doesn't necessarily mean anything other than "Gods from before the current pantheon", of course, but it certainly has a strong connotation in popular culture.

Well, there's also Dibella's quest, which deals with the Forsworn. They have all those mutilated human remains on a shrine of Dibella. Not sure if this has much to do with Hagravens but they seem interconnected.

Are Spriggans really lesser Aedra related to Kyne? All this time I thought we had not ever enounted lesser Aedra. Huge news for me, please confirm.

We've got no conformation they are "lesser Aedra", we're mainly using that as a word as we've got nothing else to describe their relationship to Kyne (We don't know much about it anyway).

Could they not be aspects of Kyne, as Wulf in Morrowind was an aspect of Talos?

Possible, but Wulf and the rest were cheeky small references that helped you, and Spriggans are, for the most part, hostile. Also, I imagine avatars are difficult for the Aedra to create and this is why we've seen so few - where as Spriggans are all over the place.

And the Spriggans no longer glow after you kill them. It seems to me that their soul leaves, and goes somewhere in particular. Like a Daedra but on mundus.