I wonder if this "First Dragonborn" was Konahrik then. His name does mean Warlord, and it would explain why his mask (and presumably a portion of his power) was sealed away by the other priests.
Well, the trailer comments that the First Dragonborn ruled over
Solstheim, not Skyrim. And I sort of assumed from the way things are presented at the Bronjunaar Sanctuary that Konahrik would have ruled over Skyrim.
So I suspect it's a new character.
Isn't this weird that this particular dragon priest did not serve dragons? By the standards of the cult being a dragonborn would be blasphemous.
I'm not sure being a Dragonborn would be "blasphemy" to the Cult... unless you mean the sort of Dragonborn who goes around killing dragons, i.e. our character in Skyrim.
Maybe a Dragonborn would have been revered by the Cult, for all we know. Maybe this character rose through the ranks of the Cult until he reached the point where he could usurp authority from the dragons with impunity, and promptly did so. And hasn't it been said that the Dragon Priests in Skyrim acted semi-independently from the dragons anyway (that is, they were the ones who established a tyrannical government, not necessarily the dragons themselves)?