1. That's only a reason to bring the (ex)Inquisitor back as an NPC.
2. That means more work for BioWare. And they HATE work. More likely they will just place each class of (x)Inqz into one of three groups; sword and wearable shield for warriors, light staff attacks for mages, and crossbow arm attachment for rogues.
3. They will. Solas may have done something to Flemeth so that the compulsion can't be used against him, regardless of who drank. Though the Well's knowledge is still accessible. But I don't see the compulsion being brought back. It would be too big of a plot to work around. Also, Mythal and Solas seem to be on the same side. It would be different if they opposed each other.
4. Your decisions ceased to matter when Trespasser was released.
5. This will always be the case however. Whether it's Hawke and company fleeing the Fifth Blight, or the Herald interrupting Corypheus' plans during the mage/templar war, the previous events have an effect on the current ones. The only time it won't is if we move to a different age. In which case it will no longer be called Dragon Age.
6. What if they do one origin story for a specific character, like they did for Hawke? Also, a native born Tevinter might not be educated in the ways of Tevinter, if that person began as a slave or something. But even if someone prestigious, how the character gathers information on Tevinter is all that is important. So instead of asking questions, he or she will just find scrolls/books or overhear conversations.
7. Not sure what the Qun has to do with anything. At the least, a certain group of Qunari attacked the Inquisitor. A Tevinter on the other hand would have been at war with the Qunari for ages. So a Tevinter would be more connected in this way. As for connection to other characters, that's just going back to points 1 and 5.
8. Not sure how you played your Inquisitor, but mine has plenty of personality. Trespasser was a detriment to character development because it forced the Inquisitor to behave in ways that advanced a poorly written plot.
9. Hawke was introduced to the Inquisitor without any problems. The same could be done with the (ex)Inquisitor. Also, bringing back the (x)In might make for an interesting continuation, but it would also demand the return of various characters from Inquisition, particularly each romance option. Having the (x)In play the role of a guide to a new protag would easily cut down on the level of involvement each of those returning characters would need to show. If they (x)In shows up and leaves somewhere for a while, the interactions with the romance option can be short, and most of it can be described off-screen. This is what BioWare will likely go with.
10. And what would be more overlapping than the SAME hero from the last game? Better to just have a new protag who can be a hero or who can be made into an anti-hero.
Also, here's something that occurred to me while reading and replying. What about new players to the series? How are they just going to pick up DA4 and start playing with the (ex)Inquisitor who was already in another game and has a range of varied pasts? Or would they be given a default world state? In which case, what's the difference between that and a new pre-determined protagonist?