I posted this in another thread earlier tonight, but it fits here. This is pretty long, but this is what I would personally enjoy:
1) For the first story DLC, it seems obvious to me that we'll be dealing with the Wardens. More importantly, one big DA tradition was not in Inquisition: The Deep Roads. We never went there (even Varric comments that we weren't in the Deep Roads, but too close for comfort) and the game creators almost always find an excuse to go there. Come to think of it, the only times the player hasn't gone to the Deep Roads at all was in MotA and now in Inquisition. I'm almost positive that we'll end up in the Deep Roads. I think we will end up at Weisshaupt for sure and probably going into the Deep Roads from there.
According to the lore, the Anderfels is a rather blasted place, much worse than the Western Approach. In some areas, corpses don't decay with how poisoned the area is. I guess a few hundred year long Blight did a number on the land. Anyway, recycling a small part of the Western Approach would work the best for an area to put Weisshaupt in. There isn't a whole lot there, other than sand and cliffs. It's even already got some Warden icons here and there. So the devs wouldn't have to set up an entire zone, only Weisshaupt and a bit of the surrounding area. Throw in a few more griffon statues, some preserved-looking corpses and it'll fit with the lore. I know I've been wanting to see the Fortress since the Fade sequence in Origins. 
2) More dwarven lore, probably tying into the above. The Legion of the Dead, or the dwarves of Kal Sharok might have a few stories of their own when it comes to weird Darkspawn and histories with the Wardens. We did get the chance to work with that city with the War Table, so they could pop up again in a more concrete fashion.
Bringing it back to the Wardens, there have been plenty of dwarven Grey Wardens throughout Thedas' history that also might have very old stories as to what's going on. Perhaps more about the Old Gods, and why there are ruins scattered around with a mix of both elven and dwarven styles in the Deep Roads. I do think they are related, after paying close attention to Solas' behavior this time around.
3) Speaking of which, I think that Solas' story will be the final DLC, just like with Witch Hunt. I suspect that, since the male GWs got to walk through the Eluvian with a romanced Morrigan, I have a feeling that something similar will happen with the Solas romance. I also think that, just like with Morrigan, you'll get a chance (try) to stab him, chew him out or otherwise wound him for creating the whole mess. No offense, Solas fans.
I also think we'll see some of the Mythal/Solas situation resolved by the end. And possibly something that will create a more satisfactory sequel hook than the post credit ending. I also think that this DLC will involve Morrigan somehow. I'm starting to believe that her merging with Mythal (not Flemeth) is her fate that the writers have in store for Morrigan.
It'll be regardless of their choice about the Well or the OGB Kieran, or the romance status with the GW, or any of that. There would be alternate dialogue to address those things, but it wouldn't change a huge amount of a potential story. I think that Solas will want to let Mythal be reborn, and would welcome another of..."his people" in any incarnation to help achieve his goals for the elves. We'll see how that works out. <shrug>
I don't think Flemeth is going to take her over, nor do I think Morrigan would stop being herself. This merging would be voluntary, to ensure that old magic and knowledge wasn't lost. That is Morrigan's MO, after all. While I think Mythal lives on through some sort of weird soul/body sharing thing with Solas at the moment, I believe that Flemeth as the players and characters knew her is dead.
That's my current DLC choice list.