My other issue with bringing the Inquisitor back as the PC is that doing so would require dragging far more returning characters back with them to make sure everyone was satisfied with their relationship progression. You'd need to bring back everyone who was romanceable or at least a scene or you'd have an uproar.
No one was satisfied with how things played out in ME2 going into ME1 when it came to how they handled the pre-existing romances. Hell, there was a lot of outcry over the way things were handled going from DAO to DAA with Leliana, Zevran, and Alistair not coming back. People complained about Hawke's LI not being with them for their cameo in DAI.
So either you cause a riot by excluding them except in the form of a letter, or you fill the game with a bunch of redundant scenes of characters who only show up if they were romanced, or you give me another game with the same damned cast.
Give me a new protagonist and make it so that the Inquisitor and Dorian are the only returning characters and I'll be ecstatic.
Probably because it will never be made. BioWare will make the inquisitor a crappy NPC advisor, the protagonist will be a random inquisition agent because "Solas doesn't know" them and will do NPC-quisitor's bidding to gather forces or magical McGuffins to defeat Solas. I don't understand what you assume can happen in DA:I with a new protagonist. Based on what we've been given in Trespasser, the things that can happen are very limited.
That would be worse case scenario, but keep in mind, most likely scenario for your side of the argument is that the Inquisitor gathers up a group of completely random new party members because "Solas doesn't know them" and gathers forces or magical McGuffins to defeat him, and after he's defeated you have the option of killing or sparing him.
What do I imagine happening with a new protagonist, best case scenario, if I were writing the whole thing? If you'd like I could write you a rough draft pitch.