@Dean_the_Young:
Ah, I see. Well, it is a political counterpart - we do have mages ruled by non-mages, and what we're discussing is a state of things where non-mages would be ruled by mages. That's what I meant.
I do think it's plausible in the future- especially a game in/of Tevinter. I just also think it would warrant being an ideology that isn't limited to one player class.
Also, as someone else already pointed out, with "plausible" I meant politically/philosophically plausible as a reasonably rational in-world ideology, independently from the question of whether we would actually be able to adopt it in dialogue.
Mage supremacy ideologies already exist in-world, even if they haven't been explored in detail, so I think it's perfectly reasonable. I'd be amazed if the Tevinter Circle doesn't have lessons about how Templars are constantly raping and torturing mages and using BSN-level hyperbole as a cautionary tale about what happens if mages are restricted by mundanes.
Also, I do not agree. We do have a state of non-mageborn supremacy that supports the other classes, if you want to break it down to that, so it's perfectly plausible that the counterpart for the mageborn will be made available as a theme. Of course I don't know and actually I don't care that much, and it's not what this topic was intended to be about, but it should be perfectly possible.
Anything is possible, for enough resources.
As for your specific ideology... I don't think there's a cultural opening for it to be a major one in-universe, and if it's just a fringe belief I don't think they would let the PC be a member of it. Not yet, and not unless DAI for some reason ends with a Mage Free State of some sort.
The reason I say this is because your proposed ideology, if I understand it, is emancipation rhetoric in concept. It's the sort of 'mages must rule to be free' idea that only really appeals to, well, mages defined by the Circle system, who are the only major mage group that doesn't already rule and have greater freedoms than the mundanes.
Mage supremacy we have seen and heard of elsewhere overwhelmingly focuses on the legitimacy of mage power, rather than mage rights. Tevinter basically uses a variant of 'the greater good' in justifying mage rule. Seers and shaman types tie magical powers into an exclusive sort of spiritualism. And so on.
While Southern Thedas apostates in Andrastian cultures could be a plausible group of this, it would only be as a fringe sort. Most just want to keep their heads down and out of sight, for fear of the public. Of those that we have seen and heard of with mage supremacist aspirations, it's generally of the Tevinter sort. Emancipation rhetoric would be fighting for space in its own population group and their minority of mundane sympathizers (who probably see an exclusive state for mages to rule themselves somewhere else, rather than mages ruling over mundanes here at home).
Mage Emancipation Supremacy would be a great ideology for sustaining a Mage Free State after independence, but it already fights against well established ideologies within the Circles.