K, so I'm not sure if this is an old theory or not but here it goes anyway. We know that some people say dragonborn come from Akatosh, and some say the Emperor dragonborn become dragonborn after they light the fires. I think. But Akatosh came in later and we now know Alessia isn't the first one. Miraak apparently was. So that means that someone else was sending them besides Akatosh. I don't think it was Alduin because that wouldn't make sense. Why would he send these beings in the world that could be his undoing? So another possibility is Shor. As some have said, Shor and Alduin have been enemies before, likely still are.
Once in a past Kalpa when the Greedy man tried to hide pieces of the world along with the leaper King to undo him, the other time when he and alduin's ghost fought and Wulfharth learned the shout "What Happens When You Shake the Dragon Just So", and again when Alduin came to Sovngarde. So would it not make sense for Shor to send Dragonborn? They devour the souls of Dragons. Sounds like something one would do if they were an enemy of Alduin. And our destiny was to save Sovngarde and the world (the one he made possible) from Alduin. If not Shor then who in the world did the Atmorans believe sent the Dragonborn? Again it cant be Alduin because they demonize Alduin. No one else in their pantheon makes sense.
And if it is Shor, could it not be also Akatosh as well, as far as the Cyrodillic Dragonborn go?
Discuss, and sorry if this is an old theory.