[quote=@BlackBishop]
<Snipped quote by NarcolepticSailor>

Yeah, that's what I thought. Septims were Dragonborns, gifted the Amulet of Kings to hold back the Oblivion hordes, IIRC, and their line ended with Martin.
[/quote]

Alessia and Reman both held the amulet, too, so I would assume that it wasn't limited to the Septim bloodline.

Whether the gates can even be opened is  unclear, though.  Martin declares them to be shut in the cutscene after his fight with Dragon.  Then the Shivering Isles came out - very clearly in one of the plains of Oblivion.

So I have no idea if any if the Daedra are going to invade.  It would certainly make things even more interesting - though for better or worse I could not say.