Those who played DF probably remember how Totem of Tiber Septim could be given to certain few people in Illiac Bay. However, it said that it could only be used by "Descendants of Tiber Septim" yet that made no sense back then because some, like the Ord King, could hardly be descendants of Tiber Septim.

With new lore info, could we conclude that the Totem was made so it could react to the Dragonborn? Maybe Talos thought he was the only Dragonborn and thus wrote that requirement thinking that only his children will be able to use it.
Which would also mean that: Gorthwog, Mannimarco, Zurin Arctus and the three royal families are also Dragonborn.

I'm not sure what actually constitutes being "Dragonborn". Could someone clarify that, if possible?

I think - Shezzarine.

This is the same kind of discussion as "Why was Mankar able to wear the Amulet of Kings?"

The general consensus if I recall correctly is that sometimes being kingly is more about blood, but also intent and destiny. These people walk certain paths which grant them the attributes of someone who is kingly and set above other mortals. It's a form of mantling almost. Someone like Gortwog is an agent of destiny, on some level or another, because his actions can elevate a people and a nation to a new level, which is exactly what he did and why Orcs were citizens in Morrowind and Oblivion and not just mindless monsters.

And why the PC can't use it, or at least will get stepped on.

Yeah the PC from Daggerfall is more like a "go fetch it" lackey, like the Oblivion player character.

In Morrowind and Arena you were more of a big hero.

This isn't necessarily true.

This definition would also explain how the PC in Skyrim can be a dragonborn without a Septim blood line.

So the eight factions of Daggerfall could use the totem because they deserved to be able to because they plotted for it in the first place, basically.

The way I always understood it is that their ambition and willpower caused them to be able to wield the power. They succeeded because they forced reality itself to bend to their will. They couldn't all get everything they wanted, of course. The Jills were able to mend it into a compromise of sorts. There were many winners and even more losers, such as those unfortunate nations which disappeared in a fiery blast.

No... but then again it is too convenient that there are so many descendants of Tiber Septim... isn't Mannimarco even older then him... and Zurin Arctus definitely ain't his descendant.

Now that's just a whole other can of worms.

Mannimarco learned he was a god, in the middle Dawn:

How do you think I learned my mystery? The Maruhkati Selectives showed us all the glories of the Dawn so that we might learn, simply: as above, so below.

Zurin's closer than kin. I consider the rest to be blood. Why not? Part of conquering is humping the nobility.