I know this won't be a universally popular opinion, (and, believe me, I'm quite surprised to be writing this myself) but, following the events of Trespasser, and that beautifully melancholic final conversation between the inquisitor and Solas, I'm now eager to play as the inquisitor again in Dragon Age 4.
The fact that Solas is capable of killing the inquisitor but chooses to save her instead, despite knowing that she'll do everything within her power to stop him, really hit me emotionally. That moment illustrated just how much Solas respects the inquisitor (at least in my game), and it set the stage for the best kind of rivalry: one between friends on irreconcilably diverging paths, with a tragic and, no doubt, tearful conclusion. lol
Introduce a new protagonist to track down Solas and all of that beautiful subtext is lost. Solas becomes just another Big Bad who our disconnected hero has to put down in order to prevent the apocalypse.
P.S. I also think that the inquisitor will be a much more relatable hero now that he or she isn't a "special snowflake". There's no more magic mark, no more grand, holy crusade. The "inquisitor" -- depending on the ending you choose -- is now much more like Hawke: a seasoned fighter and leader working alongside a relatively small group of trusted allies and contacts.
P.P.S. Yes, I know the Quizzy lost a hand, but -- in the increasingly insane magical melting pot that is Thedas -- I'm sure there's a way to remedy that minor inconvenience. If magic can heal serious wounds like deep sword gouges to vital organs, and bring people back from the brink of death, then there's probably a mage or two out there with a knack for regrowing limbs...
I agree about all the potential for subtext by the end of Trespasser, but I wasn't particularly emotionally affected by it- not even after having had Solas in the party nearly the entire game and having played an elf girl who, well, at least tried to romance him (apparently I missed some trigger early on). That moment just felt like a momentous opportunity to get questions answered- and was lost on Solas keeping mum (similar to the way Morrigan keeps mum or goes cryptic at the end of "The Witch Hunt").
And why can't similar subtext be obtained in DA4 with a new protagonist who is prepped by the Inqui to do what the Inqui can't (because Solas can evade your Inqui). A cameo by the Inqui could coach the new protag to either thwart Solas or reach out to him, but the new protag would have their own experiences with Solas, ones more pivotal than those the Inqui had, given that you can just as well play DAI without traveling with Solas at all or completing any of his personal quests- and thus little to no subtext at all by the end of Trespasser. If the new protag is sent to infiltrate Solas' organization there would be any number of canon experiences with Solas that all players have that make a final confrontation with him far more meaningful- and that much more meaningful with the Inqui on the side urging one thing while your new protag may want to do something else. The potential there to attain the same emotion-laden encounters (or better) eliminates any need to do so by reintroducing the Inqui.
I also agree with another post above that the Inqui is by far the least interesting of DA's protagonists thus far, so if they'd make a new protag that already had a rich pre-story and immersion factor, the relationship with Solas would be far more formative and interesting. As you said, being down to earth is more "relatable," so why force that on a fallen Herald when it's much simpler with a new protag? If they were going to go the route of a single protag, I'd rather have my Wardens with whom I had the most memorable and emotional experiences...
Quizzy: "--because I've grown terribly accustomed to feeling like a god. Death rays really are a must. The greener the better."
Things like this work entirely against Quizzy being a good DA4 protagonist- at least in the DA series. Besides the Quiz's history as a celebrity (something new DA players would have to simply pretend without having had any "subtext experiences" to draw from), there is also the fact that they're potentially 27th lvl while starting as a Lvl 1 again- which is just... lame. Not to mention the loss of all the equipment that would make no sense and- at least for me- would be somewhat insulting after all the time I had to endure crafting and grinding in DAI to make them. Playing with a new arm- although do-able- is also a narrative stretch. They didn't have to remove it, mind you- could've had Solas neutralize the Anchor or something (it was his after all): they wrote in the arm removal for a reason. There just isn't much of a case to make for continuing with the Inqui unless it's a high-level adventure going from Lvl 27 to Lvl 50 with the same equipment, same crafting system, same need to go grinding for schematics and mats, same spell and ability system... In this way my Warden would've been Lvl75 for DA4... And there goes the "relatability" factor or even the sense of a fallen superpower since you'd still be a superpower.
So just on a game mechanics level alone, a new protag is really the only way to go. And it's not so bad... Inevitably there will be a new cast of characters we'll have memorable experiences with as well, just colored by the metagame knowledge we have about Solas' DAI revelations...