RE OP, (had to delete the quotes to post this for some reason).
I agree with Tevinter, (obviously), but I feel like the Qunari will be on the major aggressive here so I think we will be largely limited to the Tevinter mainland. If we set sail, I think it will be to a major defensive stronghold somewhere in between Tevinter and Seheron.
I don't think the slave rebellion is at all baseless. Trespasser's epilogue, (or at least mine), had a lot of elves clearly following Solas and disappearing. The slave rebellion could easily be weaved into Solas' plans and/or the Qunari confilct given how appealing the Qun is to the elves and slaves of the Imperium. Either way, it's a very easy plot to have driving the reform arc.
I also think we might be dealing with an Exalted March, or at least the possibility of one. Again, I don't know how things work in other people's games, but mine had Divine Vivienne both threatening an Exalted March against Par Vollen after Bull's betrayal and attack on the Inquisitor and herself, and then the epilogue suggesting she was considering using the alliance the Inquisition had with the Qunari against the Imperium. Either way, the threat of a southern invasion against either power could be used to help drive the story, whichever way it might go.
Agreed on your thoughts on the companions. I would love Maevaris to be a companion for many reasons, most important of which would be I like playing as the badass fashionable/best aesthetically designed companion as I did with Vivienne in DAI and Morrigan in DAO, especially if our armour options suck. But, I have a feeling Maevaris is more likely to be in a similar role as you outlined for Dorian. I think Calpernia is much more likely to fulfill her role as the pro-Lucerni mage companion.
For the anti-Lucerni side, I think maybe it would be more interesting to see it from the perspective of a non-mage or a mage who isn't an Altus Magister. Maybe a mage refugee from the southern Orlais who willingly put themselves in the service of a Magister in hopes of becoming a full citizen someday?
As for on the Qunari side of things, I think we're also likely to see Rasaan as she's the Ariqun's successor. Maybe the Ariqun dies over the course of the game and Rasaan takes over? I definitely feel she'll make the Qunari much more antagonistic and more of a threat.