Reading this text, it doesn't really make the distinction that Tiber's bloodline are all Dragonborn in the same sense that the the Dovahkiin (PC), is. My interpretation of it doesn't change - the progenitors of their lines (Alessia, Reman, Tiber) may have all been Dragonborn, but I do not think their descendants were. No doubt some vestiges of that power remains, gradually distilled over time though, in my opinion. If each and every one of their lines were all Dovahkiin, I do imagine there to be quite a few scattered around Tamriel presently. By virtue of bastards et al
I get that, but then I have this for you:
http://www.imperial-library.info/content/trials-saint-alessia
http://www.imperial-library.info/category/tags/amulet-kings
Both state you need the blood of Akatosh in order to light the Dragon Fires, the first book says that they are the same thing, those two say you need it to light the dragonfires. We know that the First, second and Third Empire all had Emperors that lit the dragon fires without blood relationships to each other.
So it does not look like dragonblood is something you get from your parents, or maybe it is. But you can also get it by virtue alone.
http://www.imperial-library.info/content/arcturian-heresy-0
Well, we can still hope that in the next TES they won't change the events of Skyrim at least. Another reason why Black Marsh should be our next stop, since every story there would be rather original and wouldn't have to rely heavily on other tales from the different regions. And the Hist are basically Cthulhu in plant form, which makes everything automatically awesome.
I'd prefer Elsweyr. Just surviving in Black Marsh for anybody that's not Argonian would be a huge retcon in itself.