Hmm. I think that the next game being set in Tevinter and thus likely featuring a new protagonist does not necessarily preclude the Inquisitor from coming back as a protagonist later on. I would imagine that after the events of Trespasser, both the Inquisitor and Solas need to go to ground for a bit, and re-assess and re-consider their potential future plans. I can absolutely see the entire Inquisitor v. Solas situation go quiet for a bit, with the respective parties pooling their resources before their next move and influencing things from the shadows, so to speak, without any overt direct involvement on part of the leadership.
Tevinter plot could be a chessboard of sorts, with the new protagonist coming into contact with representatives of both camps and doing their thing, possibly choosing a resolution to the situation there that would either benefit the Inquisiton more (whatever version thereof) or Solas. The game after DA4, depending on where the setting and the story moves could then possibly feature the Inquisitor again (come on, BioWare, give us that cool magitech prosthetic arm to whoop behinds with!) Besides, I think I remember one of the devs saying they have toyed with the idea of multiple protagonists, so it is not entirely out of the question.
Ah, never mind me, just some random thoughts.
But if I am to be completely honest, I do agree that having a new protagonist each subsequent game is giving me a protagonist whiplash a bit. Speaking entirely subjectively, with each new DA game, I get this new character I become invested in, I help them go through hardships, through some good times, I am then always teased that more may very well come their way aaaaand...you never see them again. Or, as is the case with Hawke, you only see them again to have them potentially die. That is...hm. Yeah. The whiplash thing.
And I mean, it does not necessarily have to be between "have a new protagonist every single game" and "have the same protagonist every single game". What if there were three main people who rotated between installments? Each would have their respective field of expertise and each would have their theme to which they would be central. The Blights and the Warden-y things would be the domain of someone like the Warden, so games that are Blight-heavy would have someone like the Warden as their returning protagonist. Someone like Hawke, who is tied to the mage/templar/red lyrium portion of the lore, would be the returning protagonist in installments that feature mages/templars/red lyrium. The Inquisitor is all about demons/spirits and the Fade and Solas, so if the game is about demons/spirits and the Fade and Solas, the Inquisitor would be the returning protagonist in that installment. They would take turns, each would have their own story, which would form a section of the main over-arching story. This way, it is not just one person in charge of everything everywhere ever, but everybody does their thing, and you get to see the characters you became invested in again on top of that. I am not sure how doable that is, though. Again, just a purely subjective, silly thought.