I'm curious about the lore behind the Dragonborn in (Ancient) Nordic culture. There are bits of things in the game that talk about the Dragonborn existing in the ancient culture but I'm confused as to where that leaves their origins at.

I've read a lot of speculation that points at the Dragonborns' significance around the Emperors coming from Alessia instating Akatosh at the head of the Imperial pantheon and that she was the first Dragonborn Emperor and that the tie to the Amulet of Kings grants the Dragonblood through a pact or covenant of sorts as the direct bloodline of the Septims has been broken several times and an explanation was needed to remedy that.

I'm wondering about the Dragonborn that existed before this and their significance, if there is any information existing on them at all or if they even existed in the first place before Alessia. I think I've read that Ysgramor may have been one somewhere?

Kyne/Paarthurnax granted Mortals the ability to use the voice first in ancient Nordic/Atmoran culture, but where does this leave the origins of the first Dragonborn at? Was another aspect of the Dragon God of Time already at odds with his mirror-brother Alduin? Bormahu perhaps? Maybe Alessia creating Imperial Akatosh to have always existed had some effect on it? What purpose would a random fragment, non-Emperor Dragonborn even have to exist other than to prevent the Kalpa from being ended anyway?

Alessia (and Maruhk) walked so hard that They Walked Like Themselves.

Atmorans had dragon worship, so them venerating a "dragonborn" wouldn't be so out of the left field. After all, a dragonborn just has the soul of a dragon. Shouting isn't necessarily a talent unique to the dragonborn. Or rather, dragonborns don't have to be able to shout to be a dragonborn. It's only Skyrim that associates the Thu'um with Dragons. Dragonborns probably existed before Kyne gave the Voice to Men.

I'd vote that Dragon Priests say they're "dragonborn".