Personally I would prefer a story that ended in anti-climax for the Inquisitor.
Fact is that there's nothing special or magical that makes it to where the Inquisitor is the only person who can stop Solas. There's nothing but a personal grudge that not everyone bought into.
Nothing special? Whether they lost it or not, they held one of the most unique powers in Thedas at one point and there's no telling if there aren't some remnants of it left.
They've also managed to organize and lead an organization that dealt with an ancient magister darkspawn who tried to become a god, and changed the political landscape of the realm.
They have access to leaders of most known Thedas, they've scoured the world for knowledge and influence, they've searched and uncovered the secret history of Thedas at a capacity no one else has done before. Aside from challenging the ancient darkspawn they've also fought and defeated an Avvar spirit-god, discovered Titans and traveled through eluvians to find out about the lost world of the elves and rebellion of the Dread Wolf.
...And on top of this, the creator of the frikking Veil itself either considers them a nemesis or his way for redemption.
You're fooling yourself if you think that the story hasn't made Inquisitor special in many respects.
Joss Whedon is a very big influence on Bioware's writing for Dragon Age and one of his big things was deconstructing the idea of "I'm the only one allowed to defeat you!" The inquisitor has declared that they'll be the one to stop/redeem Solas, Solas has marked the Inquisitor as his rival, but the fact is, that means all of jack to the rest of the world.
The rest of the world is endangered by a man to whom Inquisitor either means a lot, or at least Inquisitor knows them on a level very few other know him. If you think that amounts to nothing from either the world's or storytelling perspective, then boy - I've got news for you...
You don't set up an active relationship between a protagonist and one of the most important persons in whole of Thedas through the entire duration of DAI, imply in the epilogue and the world-state Keep that Inquisitor's new mission is to stop Solas and then just... replace them.
That's like building an entire, multi-chapter saga about Avengers, only for the last movie for them to be replaced with Fantastic Four for no reason.
I don't think I should tell you how much of a bad idea that is.
Besides - I've already touched upon this in one of my older posts, so I'll just paste it here:
"DA would still remain true to is core concept of telling the story of the world, rather than one protagonist, if the story was written in a way that would make the fate of the world (and Inquisitor) hinge on decisions of DA4 hero.
It can be done. In fact, it HAS been done, in many different ways: decisions made in previous games directly influence how events play out in Inquisition. And, of course, there's always the intentional grand irony of DAI's story: that it was Corypheus who, in all his madness and arrogance, gave the world another chance, by disrupting initial plans Solas had. It is thanks to him that Inquisition - and Inquisitor - actually rose to power.
Or take TES: Oblivion for example. That story was a story of Martin Septim - it was he who eventually stopped the main baddie. But without Champion of Cyrodiil he wouldn't be able to do anything. The Champion was the connective tissue between Martin Septim and his destiny.
So can the main PC of DA4 shape the world and enable things through his or her actions and they can feel no less important than Inquisitor."
The fact remains that a new protagonist can be important WITH Inquisitor still being retained as a major player in the narrative. Not only it would be perfectly within the lines of what DA creators have set to do (tell the story of Thedas through multiple eyes and interconnect their story in some important way), it was already done at least once in this story - without Hawke, neither Corypheus would be released, nor Inquisitor would know of a Warden in Crestwood, the contact with whom enables them to stop Wardens and Nightmare from creating the demon army for Cory.
Anyone of sufficient will and capability can bring Solas down. If anything, it would be more appropriate that he is defeated/redeemed by someone other than the Inquisitor. His problem is that he doesn't see the people of modern Thedas as having any worth, with the Inquisitor being the only one to prove him wrong. Having someone else prove him wrong would go further to show that the people of modern Thedas are worth preserving, rather than saying "they're all worthless except for this guy."
No - Solas certainly views approved Quizzy as probably one of the brightest souls of this age, but he doesn't say anything that suggests that he views only Inquisitor as a person. He states verbatim "you have shown that this world has value" and "you were people and you deserved better".
And while I agree that bringing more people together to convince Solas to change his plan is probably what will happen, it still remains Quizzy's job to bring all those people together. Whether someone else kills or redeems him, eventually Quizzes will take credit, simply by the virtue of being ones who instilled either doubt and hope in Solas, and back in the time when we were allowed to know and relate to him in a way no other person will likely be able to (seeing how Inquisitor's story and their role in universe in many respects resembles that of Solas as well).
Therefore I don't see how it's possible to write a story of redeeming/ending Solas without them, simply from a way the story is shaped now.
Not to mention the simple fact that a non-Inquisitor protagonist has a built in advantage; Solas has tunnel vision directed somewhere else. Hell, that was my interpretation of the stinger; "Solas knows us. We can't go after him. So we're going to find someone he doesn't know to take him down for us." That's what I got from that scene.
The Warden didn't get to deal with Morrigan and Flemeth's conclusion. Hawke didn't get to take down Corypheus or factor into the end of the Mage/Templar war. It falls perfectly within Bioware's pattern that the Inquisitor will not be directly responsible for Solas' downfall.
.... How did you come to a conclusion to "we can't go after him?" when the last scene tells something completely different? Leliana warns that he knows them and their current strategies, to which Cassandra responds "we will have to be careful" and Inquisitor ending with "we will have to fight people he doesn't know."
Also - neither Warden nor Hawke got an epilogue or keep choices that suggest that this time the protagonist sill has a pretty active role in future story. Heck, Warden can die (and mostly did) at the very end of DAO, which in itself cements their fate as heroes who do not return (and the devs keep repeating that he won't be back, which is something they're yet to do with Quizzy).
In fact, whether they can die, or leave to parts unknown is actually a pretty good indicator of a hero returning later - we know that Warden won't now, and Hawke can die in DAI, which indicates that the won't be back at any larger role in future installments.
Quizzy on the other hand? They don't die. They don't travel to parts unknown. Each Inquisitor, no matter how good or bad at their job, or being friends with others all and up in pretty much the same spot - alive, with no hand, but remaining resources and important people, secretly conspiring what to do to stop Solas and planning to visit Tevinter in one way or another.
That in itself speaks volumes of their possible future involvement.