It fits in with Sandal's crazy talk in DA:2 about everything being as it was.
The elven gods and old gods have to be related. I like the whole war of the gods idea.
Yes, I've always been intrigued about what Sandal meant. At the time I presumed he meant dwarves will regain a connection to the fade. A lot has since been made about 'when he rises' being Corypheus, but it could have been Solas waking up.
As for the elven gods and old gods, I have a whole theory (of course I do!).
*rolls up sleeves*
We know elven gods could turn into dragons, and the dragon form was considered sacred. We know there were 7 elven gods sealed away (in the heavens, apparently), and legend has it that there were also an unknown number of 'forgotten ones' sealed away (in the abyss) at the same time. We also know that there are 7 old gods, and they all happen to be sealed away underground, in the void, or the abyss, or whatever you want to call it. These 7 old gods are dragons.
I have 2 theories, and I like the second one the most, but both are possible, imo.
1) The 7 old gods were other elven gods whose names were stricken from the elven pantheon (hence being called the forgotten ones) because they went in a different direction ideologically and philosophically to the elven gods we know about. They favoured their dragon form, perhaps a dragon cult of sorts grew up around them, and threatened the power of the other 7 elven gods, who relied on slavery and seemingly ruled quite strictly. The two sides warred, and Fen'harel found a way to seal them all away to stop things getting more out of hand.
2) The power responsible for allowing the 7 elven gods to turn into dragons was actually something separate to them, yet inside them. In a way, they had a duality, with their 'normal' elven soul and a dragon aspect, that helped them be as powerful as they were. Andruil it seems had contact with the taint when she would travel into the void, and it seems she was corrupted by it, at least in part. She also seems to have spread that corruption to the world above ground (her mosaic in the Temple of Mythal says her lands were plagued). She used to go into the void to hunt 'the forgotten ones' - which I think was the elven gods in their dragon form, because I don't think she thought it should be reserved for them alone, and she didn't like their abuses of power. Mythal took her knowledge of how to get to the void. I think this kickstarted a war between them. Dirthamen probably was on the side of Andruil (it was one of Dirthamen's disciples who wanted to take on the dragon form), Andruil probably killed Mythal... it was likely a massively f**ked up war, and very messy. I expect a lot of innocent people got caught up in it and used as weapons. I think each of the gods (except Mythal, who was killed, and Fen'harel, for reasons I don't know) eventually contracted the taint, and it was driving them even more mad, fuelling the war further. Fen'harel wanted to stop the fighting before it got out of hand. He found a way to separate out the taint from the elven gods, but it took ripped out the dragon aspect of their soul - which by this time was fully integrated, and meant they were left unwhole. He didn't know how to deal with that, so he sealed them away separately, hoping to find a solution down the line. Hence we have 7 elven gods sealed in the heavens, and 7 old gods (their dragon aspects) sealed in the void/abyss/underground.
Now, that means those dragons are already tainted. We've learned a little about dragons and the taint in this game. One thing we learn is that in the Western Approach, the dragons who were studied had bits of the taint inside them, but it was protected, sheathed in something away from the rest of their bodies. I think this is how we can think of the dragon aspects of the gods and why when Fen'harel separated them, the dragons took the tainted parts with them. The old gods/dragon aspects already being tainted also means that we think about blights the wrong way. Darkspawn don't taint the old gods when they find them and trigger a blight; they are drawn to the old gods because they are massive and powerful loci of the taint, and when they find them they're setting free a dragon that is already corrupted, which they then follow as it is now free to rampage across the world, likely fuelled by a blind rage for a war it can't even remember (since it's not a whole being anymore). This is why Solas is so angry and grumpy about the Grey Wardens. For one, they are slowly killing parts of his brethren, stopping him from ever being able to reunite their souls to be whole again. Second, they think that when they've killed the last of the old gods there will be no more blights, but in fact all their death will mean is darkspawn won't have that central figure to follow anymore, and there will be a perma-blight of sorts, as they amble mindlessly in all directions. That might be easier to control in the long run, or it might make things harder, I don't know. But it won't cure the taint, and it won't remove the threat.
I think the Black City is Artlathen, and that it was corrupted, in the same way Andruil's lands were, during the fighting, and that Fen'harel sealed it away when he sealed the gods away. I think all of this is why he created the veil - it was a byproduct of doing such a massive, impactful thing, it was the only way he could ensure it would work. It must have pained him greatly to do that. And it must weigh on him terribly now. But I suppose at the time he thought he had no choice if he wanted to stop the warring before they killed each other and utterly ruined their society, destroyed their history, etc. Well, that whole thing didn't turn out that great for him in the end, which is why he's such a grumpy pants now. But how on earth do you go about solving a problem like that? I suppose having just wakened from his long sleep, and not having enough power to attempt it himself, out of desperation he thought Cory - the only living thing he knows of that has managed to walk in the fade in his physical form - might be the one to help him. He was clearly duped, or at any rate got that whole thing spectacularly wrong. He must feel like hell right now. The breach, on the face of it, might achieve what he wanted - no more veil, the fade and mortal world as one - but it was done in such a violent way it was ripping spirits through without their consent, turning them into demons, and it was done in such a way that would threaten the stability of all of it, fade and mortal world. There's a right way to do it, and that's what he's looking for. Whether he can achieve it or not is another matter - but I suspect the main lead and hope he has for that now lies in the anchor.