Mmm I've two other options: one concerning the translation of costumes from a culture to another, and the second about dragons and dragons' blood.
Note how in our own history so many costumes passed from a culture to the next one, or to a close one influencing it. For example the roman empire: it influenced the nearby cultures. Also the Hellenic one. And what always happen in the beginning? The uprising culture seeks to imitate the one that thinks is the best and his costumes (Hellenic->Roman, Roman->Medieval peoples; ecc ecc). So it is possible that the game developers have thought that way creating thedas: Elvenhan->Tevinter and Dwarven empire; Tevinter->kingdoms from fractured tevinter and so on. If in Elvenhan there was a practice similar to slavery the tev adapted this practice to their own culture. And later, with fenris, I suppose the magister were trying to create a better form of vallaslin, with lyrium that is naturally magical. Vallaslin are made with blood, and doesn't have magical properties but maybe they had in arlathan because the elves were MAGICAL being.. and so their blood? In fact, dragon blood too (immortal beings) have magical properties (reaver; genesis of the qunari people; etc). More to say is that the tatoo practice to sign lower classes could have been passed from arlathan to the dwarves, thought the use is different, as different was the use of slavery in tevinter (every cultures adopted some costumes and have it their ways, no? Points of view).
One of these practice is also blood magic, that was not a dogma in arlathan, or at least Solas seems to have said that. And this is strange cause the only class of magic that grant power over other beings is welcomed by the god of rebellion. The same god says also that is the use of that magic to be condemned. So maybe he was referring to blood magic binding elves to their gods through vallaslin.
In the end, flemeth is rather interested in Urthemiel soul and eventually absorbed it. And Flemeth/Mythal grant the power to shape shifting into dragon form. In fact mythal is represented with two dragon wings. Archdemons (corrupted dragons) can do the body-hop and flemeth too. All the old gods are maybe the powerful and immortal souls of dragons. The elven pantheon is composed by the creators (as flemeth is) and the forgottens. Maybe the forgottens are the dragons that had eventually becomes the archdemons. They were cast to an abyss, in fact

and the creators were sealed in the fade. The funny thing melting the andraste-maker story and this of the elven pantheon..? Fen'harel=the maker ahahahah don't think so, but it is funny