If you join the Red Jennies you get some sort of crossbow arm in the epilogue. Otherwise the devs could just have the Inquisitor get some sort of golem arm, there's even a left handed metal arm in the magical store room in the Darvaraad.
That said, I think next game it'll be a new, Tevinter PC.
Wait, there's a metal arm in the Darvaraad?
I didn't notice that. Got screenies?
Maybe over the next few years we start research into getting an arm replacement...Oh man, that sounds so neat.
My thing is, do we think in DA4 we will have the chance to choose between the Inquisitor, and a new character? Not everyone will have played DA Inquisition. It would be silly to force them to use a character they never had. Would it maybe be one of the Inquisitor's recruits? Hrrrrmmgrrr.
I also disagree with the idea that being one-armed renders all classes useless. As a mage, I'm probably perfectly fine using magic with one arm.