I just finished Descent and I've read a lot of critiques to the DLC as it left more questions (even more than those we already had) rather than answers.
I don't think we should judge the DLC just yet, as it is definitely a sneak peek into the next game's plot/ involvement. I have every confidence that Valta will be an important character in the next game, or even perhaps a companion. The mention of the absence of the Wardens also hint to some issue they'll be having in the future (like in the Epilogue).
Having content that leaves you with questions is not necessarily a bad thing. I've always entertained the notion that mystery is a powerful driving force to push forward the narrative as the player will do its utmost to try to find an answer to it. It is also probably one of the reasons Act I of DA:I is so strong, we're still trying to piece together what exactly went down at the Temple of Sacred Ashes and who, if anybody, could've been behind it. In many cases even the pursuit of this truth is better than the revelations themselves. But that's the real issue I have with The Descent I think, you're not allowed to pursue anything, do anything, you're simply along for the ride.
I can understand that the scope of what you can do may be limited but while playing The Descent there are some situations that scream choice so loud I was dumbfounded when none appear. Most of them come down to common sense. Case in point, Valta is affected by the Titan and is able to use magic. Her demeanour, actions, and responses are eerily reminiscent of the situation with Corypheus in Legacy, a situation both the Inquisitor and Varric are aware of. How is it then that no flags are raised? Surely some Inquisitors (if not most) would insist on having Valta return to Orzammar or even Skyhold to determine if she's okay? Surely Orzammar would be interested in knowing they have another civilization on their back yard? Surely it would make sense to have a few organized expeditions investigate the nature of this civilization and what answers (or questions) it may offer? When this (reasonable) pursuit is curtailed by external factors it can only lead to frustration.
A different situation would be if you were to tell me this is Chapter 1 of a X-part (not Roman numerals) expansion. If there had been an expectation that that would be the case then, while some of the above issues would remain, there's the implicit promise that this is not the end of your adventures and the pursuit of "the truth" is merely delayed, not stopped altogether. In fact, I have often wondered why not release DLC in such a fashion, following the rulebook of the episodic game model. I believe it would definitely keep people more invested while allowing BioWare the freedom of telling a story over multiples pieces of DLC. But I'm straying off my point.
The possibility that this DLC is a sneak peek of what's to come in future instalments is not, I believe, a point in its favour. Personally, I don't think DA:I benefited in the slightest by having Corypheus as the main antagonist simply because he appeared in Legacy. He could've been replaced by any other villain and it would've been the same, maybe even better depending on the circumstances. As far as we know The Descent is a self-contained story and I will treat it thus. Otherwise it would be akin to suggesting not to judge DA:I until all DLC had been released. I'll repeat it, there has been no expectation that The Descent is part of a larger DLC story-arc so I have no reason to treat it as such.
Valta could be a companion in DA4 or DA5. The Titans could play a larger role then. I don't deny this nor am I against it, that is not the issue. The issue I have with The Descent insofar as choices are concerned is that in the span of a conversation there were many (reasonable) choices to be made but none were given. That any option to investigate in the future was quickly taken away. And that Orzammar (everyone really) will remain as oblivious as ever as to what went down. That's not good, because then you find yourself you no longer are the Inquisitor in DA:I. You're Hawke and you're playing DA2.