Leaving it up to the player sounds fine to me.
Your goal can range anywhere from laying waste to the Imperium, getting the hell out of it, or becoming the next Archon.
I know I won't have a lot of company in this opinion, but I like the idea of starting out as a slave and earning your freedom.
agreed.
It is also why I found it interesting that I ended up setting my origin/prolouge idea in each of the classes.ie one origin in slave, saporati, Laetern, Altus, and ambassador(dwarven). This is good as it allows for different goals to be created.
an elf who hates slavery might want to lay waste to it, or rise up against it. My altus may wish to reform it but make the tevinter imperium even grand, my leatern elf mage might be interested solely in power. My Saporati might not care and seek to just stregthen the empire.
Those were just random goals. So having more goals is good.
Me, yes. I like my characters to have a nice past/life.
there isn't much story in starting as a magister
but that doesn't mean you can't start in power, ala human noble winding up as the co-ruler of fereldan. The story of an altus rising to magister to arachon can be compelling.