I wonder how he weighs the two priorities. He seems a lot more certain about hating the Qun than his apocalypse plan. If he's still waffling, it might be tempting to keep saying "oh, but I have to stop the Qunari first." Cleaning the house to justify procrastinating on taxes.
I think he'd focus on the Qunari. Not only have they gained an understanding of the Eluvian network, something which Solas was reliant on, but they may also have knowledge of other ancient elven magic which could cause him trouble.
Add to that the Qun is the most anti-magic code there is. If Solas' plan leads to the world becoming magical, then the Qun is completely incompatible with that, and allowing it to gain a foothold in the rest of Thedas would be awful for his plans.
The only way he'd ignore them is if tearing down the veil really does just kill everyone, then there will be no followers of the Qun in any case.
As for the OP, I think Solas is going to be the primary antagonist, so working with him is something that shouldn't happen. Sure, if you like Solas, like I do, working with him would seem cool, but that's essentially what we got in Trespasser, and I think having to face him as an enemy is a better story choice for all players, given that he's pretty much the no.1 threat to the world.
Also, it's been established that the blight begun when the Magisters Sidereal entered the Black City. It's not something that existed before then, so the idea that the Dwarven empire is a containment rune for the blight, which didn't exist until Tevinter were at their height - well after the establishment of the Dwarvern empire and the construction of the Deep Roads - is inconsistent with the lore of DA. While red lyrium was found in the Primeval Thaig, as Red Lyrium is not naturally occurring, it's presence can be explained by the thaig containing normal lyrium, and then becoming infected with the blight long after the dwarves there died. And the presence of Wraiths could also explain why the Primeval Thaig has not been destroyed by darkspawn.
I also don't see how the Titans can erase the blight when they themselves are susceptible to it.
And setting the majority of a DA game in the deep roads is most definitely something I do not want to see.
The more likely origin of the blight was that, after killing Mythal, the Evanuris were cursed - either my Mythal or by nature itself - and when the Magisters entered the Golden City they unleashed the curse, and brought it with them to Thedas.
I'm not sure what is likely or what the devs are planning, but this is very much what I would like. DA4 to get that Solas plot (ugh) and the Tevinter reform/revolution thing out of the way while setting up a new Qunari invasion. Seriously, that has been the most foreshadowed event in all three games so far. Do people really want it to be dealt with in a game that's already going to be packed with two other major plots? This way, DA4 would end with the beginning of the Qunari invasion, which would be taking advantage of a Tevinter debilitated by Solas and internal issues.
Then DA5 would be a naval adventure set a long the coast of Rivain and Antiva, maybe even Seheron and Par Vollen. The backdrop is perfectly set up for such a game. The last invasion was defeated at the Battle of the Nocen Sea, and the Felicisima Armada played a major role in the defeat of the superior Qunari navy. Cue pirate game. It's so perfect that I would actually regard it as a missed opportunity if they didn't do something along those lines.
I see no reason why both a Qunari invasion and Solas' attempt to destroy the veil couldn't both be dealt with in the next game. It's not like Dragon age games are short affairs. Even a quick playthrough can be 40 hours long.
I don't want to see too many dangling plot threads after DA4. DA2 and DAI have already had so much set up, it'll be like Lost - too busy setting up future storylines to effectively conclude what has already been introduced.
In any case, I can see DA4 being the end of the series, and then having a reboot, a remake of Origins, or a new series set far off in the future.
Either way, after Trespasser I want the Qunari to attack next game - if not in the main game, then in a meaty DLC expansion the size of Awakenings. One way the games have improved since Origins is they've become more about the politics and the mysteries of the world, not just about killing darkspawn. I think focussing a single game on the Qunari attacking would feel like a step backwards in that regard, it would be a simplistic plot in a series where the plots have become more complex, for the better.
And a Qunari attacking plot doesn't have the same potential for choice that the Solas plot would. With Solas, we may well face the choice ourselves: tear down the veil, or perhaps strengthen it forever, killing off magic completely? Both with massive repercussions for the world.