So Dominant is probably what the summoners are called. Seeing as Ifrit later pretty much possessed Joshua, I think it’s safe to say these summoners have a much closer connection to the Eikons than any other. Perhaps they sign a pack with one and then they can manifest using their body. Here‘a where the Attack on Titan parallels come in, where the Dominants can pretty much just transform into their Eikons and fight.

And yeah, I think Joshua is the main antagonist, but I also kinda expect there to be a Ramza/Delita-like dynamic. Friends who are torn apart by war as they find themselves on opposite sides of the barricade. Joshua being a noble right in the middle of a revolution is also kind of a mirror to what happened to Delita. One was a commoner who had to face the reality that he will never be recognized as fully human by those in power unless he seizes that power himself, while the other is privileged from birth and loses everything to a class they didn’t even know were against them for justified reasons. Both lose their innocence and both want justice.