I would point out that Orlais already has slavery.
Orlais does not have slavery, it has oppression. There is a difference.
Besides the alternative to the Circle system is one where mages are allowed to rule themselves, I'm not advocating a Mage government, I question why people think the Chantry or Templars would do a better job of regulating mages than the mages themselves.
And why you think that any group with special powers governing themselves is a good idea, I can't imagine. That's like saying the military should be autonomous and not under the authority of the civilian government. It's going to lead to serious trouble.
To give mages the ability to manage themselves autonomously is to invite men like Uldred and Jowan to gain power within the ranks of mages with no oversight. Now, instead of causing chaos and destruction while under the watch of the Templars, men and women who are specifically trained to handle(or at least contain) such situations, you're going to give the power over such situations to men and women just as vulnerable to such temptation?
Regardless 'regulating' mages doesn't make their powers disappear
It can't be argued that the power of mages will ever disappear under regulation. If their powers did, then there would be no need for regulation. That's a circular reasoning, and it's nothing close to what I'm saying. So I'm not sure who you're debating that with, but it isn't me. If mages are to be regulated(and I think they should), it must be permanent. Like it or not, mages will always have power and will never live a normal life. The power they wield and the demons that want to exploit it remove that possibility. It sucks, but there you have it.
you're just relying on those governing them not to misuse their powers.
The exact same thing can be argued for this "mages governing themselves" scenario. You're putting an awful lot of trust in the mages to watch over their own with diligence and integrity. Such trust is unwarranted and dangerous, in my view. Sooner or later, the mages will start to overlook things. And you'll either end up with a new Tevinter or open war between mages and anyone who stands against their ambitions.
There is little difference between a corrupt Mage government and a corrupt government with control over its mages.
I must heartily disagree.
In the first case, it is the commonfolk who suffer. Slaves, servants, anyone who is unfortunate enough to be born without the gift of magic or enough influence to be useful to those who are. Easily the vast majority of people living under the authority of such a government. Hands-down the worst form of government in Thedas, including the Qun.
In the second case, it is anyone who is unfortunate enough to be born a mage who will suffer. Easily a smaller population, and one that is decidedly more dangerous. Ideally their fate should not be that of a prisoner with no chance of parole, but if that is what it takes to ensure the safety of the common people, I think it's an easy choice to make. If your government is corrupt(the natural progression of most governmental authority, I would argue), I think that the least amount of people should live under that corruption and the most amount of people should be given protection by it.
Now don't start to peg me as one who hates mages and thinks that they should be locked up forever. I don't. I think the Circle has its problems and should be reformed. But to throw it out entirely is to put the mages in charge of themselves, and that is a scenario that I would never want to see happening if I lived in Thedas. This is because such a scenario depends on the mages living and acting altruistically and not carrying out selfish desires and ambitions. For good. The chances of such a scenario occurring in Thedas are incredibly remote, IMO.
If I were a commoner in Thedas, the last place I would want to live is a place where mages are in charge, corrupt or no. I'd prefer the Qun to such a place(and I say this as someone who reviles the Qun with a passion).