The thing about doing prequels is that the choices provided in the prequel will need to be linear and can't, in no way, contradict nor alter any events that happened in DA:O to Inquisition, and also other side-lore and events that don't take place in the games. Basically, it can't alter anything we already know, and that isn't fun. Prequels are a bad idea in these sort of games, because there's too many variables to take into account especially when the universe is larger than it was at the start. What has been mentioned before DA:O needs to stay that way to avoid what I mentioned above.
To be fair, the DA games have become more and more streamlining as we go, so we already are at a place where no matter how differently you approach a problem, the final outcome is always the same. Your Warden did the dark ritual or the ultimate sacrifice? Irrelevant, the Warden is gone either way and Kieran doesn't matter anymore. Your Hawke supported the mage rebellion or helped the Templars stop it? Irrelevant, the war happened anyway and Hawke's role in it was peripheral at best. Your Inquisitor supported the mages or the Templars? Irrelevant, the Circles have been rebuilt into Colleges anyway. Your Divine was Leliana, Cassandra or Vivienne? Irrelevant, she's always named Divine Victoria, and her term is always met with resistance and controversy. We could factor a thousand different little details into each of our playthroughs, but at the end of the day the world has largely come to the same place for all of us.
So at this point, a game set two hundred years in the past or two hundred in the future would use the same principle: lots and lots of smaller variables, generally same larger outcome for everyone, so it doesn't contradict the overarching lore of the world. History has been told; this Divine did this thing in this year and the outcome was that - but what else aren't we being told? Why did she do this, who were her allies, who were her enemies, did some big hero support her and that's why that response, did some big hero oppose her and that's why that response, etc. There's a lot of story you can work into historical events giving players all sorts of choices without changing much of the final outcome.