Well, I would be more comfortable with using mostly OCs since I don't want to risk butchering the characterization. But I'm not against having canon characters. That said it depends on the plot you have planned and how close it would follow the games' plots.

I mean, we could have the RP set in the Orlesian civil war and in that case OCs would be pretty much a given (unless we decide to go for Celene and Gaspard or other NPCs) and even then the Civil War hasn't received much attention so we would have to fill in the details (I might be wrong of course, I haven't played DA:I or read the novels). On the other hand we if we're going to follow the main plot of the games (The Blight, the Kirkwall fuck-up) the presence of canon characters would be pretty necessary, even if we just play as people "caught in the crossfire".

So in short I suggest a mixture between OCs and canon characters, the ratio depending on how you wanna take this forward