So I'd like to address the following questions: Could the Dwemer return? And if yes, should they? and If they did return, should they be a playable race?
Before we begin, I'd like to recommend the Imperial Library's http://www.imperial-library.info/content/definitive-guide-dwemer, as it offers a wonderful introduction to the race, as well as some information you might not be familiar with.
Before I address the questions, let's delve into the mystery surrounding the Disappearance of the Dwarves.
In 1E700, the Battle of Red Mountain occurred. During this time, Lord Kagrenac, Tonal Architect of the Dwemer, used his Tools on the Heart of Lorkhan. At this same moment, the entire Dwemer race vanished from the face of Tamriel.
Now, we also know the Dwemer sought to create a new god from the remains of a dead one. They harnessed the Heart of Lorkhan with the intent of placing it in Numidium (or Anumidium) the Brass God.
I'd like to bring to attention the http://www.imperial-library.info/content/final-report-trebonius, as it does a far better job than I could of explaining the exact circumstances and the explanation of what happened to the Dwemer when they disappeared. It is a long read, however, and so I'll highlight the most relevant parts:
Now, let's look at the first question: Could the Dwemer return? And if so, should they?
We know the Dwemer became the skin of Numidium. So if we are to consider the possibility of the Dwemer returning, the first question we need to ask is, where is Numidium?
The Surrender of Alinor happened in one hour, but Numidium's siege lasted from the Mythic Era until long into the Fifth. Some Mirror Logicians of the Altmer fight it still in chrysalis shells that phase in and out of Tamrielic Prime, and their brethren know nothing of their purpose unless they stare too long and break their own possipoints.
So could the Dwemer return? Anything is possible, so yes, they could. But should they return? That's a different question altogether. We can see through forum polls (such as http://www.gamesas.com/topic/1320100-which-unplayable-race-intrigues-you-the-most/) that the Dwemer are very popular among the fanbase and have always been an interest to many. It is not hard to figure out that the reason for this is directly related to the sense of mystery and intrigue they fill our minds with. The Dwemer had knowledge beyond any race of Tamriel, and they used that knowledge to build constructs the likes of which have never been seen before and have rarely been attempted to be replicated since. All 10 playable races aren't really that different from one another, and the other non-playable races are usually less advanced, uncivilized, and/or savage beasts. The Dwemer, on the other hand, were far more advanced than all the other races. If the Dwemer race were to return, we would see them the same as any Imperial or Nord or Khajiit strolling around Tamriel. The mystery would be gone. The sense of wonder would be gone.
Most likely, if the Dwemer souls were extracted from Numidium and returned to Mundus as mortals, they would go right back to doing what they were doing before, and would simply attempt this anti-creation all over again. So what would be the point? At best, they would probably be a closed-off society that the player-character would have a very minimal interaction with. At worst, they would assimilate with the other races and just seem...average. Is such a thing really worth ruining their mysterious nature and the intrigue around their seemingly endless knowledge?
And, let's be honest, for all the hype Dwemer have gotten as to how amazing their race was and the insane feats they could easily accomplish, is it even possible for Bethesda to do them justice when implemented in one of the games? I have lots of faith in Bethesda, but even I have to say it's highly unlikely. Any attempt to show us the Dwemer race in all their glory would inevitably be a disappointment.
So could the Dwemer return? Yes. Should they? No.
But let's say the Dwemer did return, for the sake of argument. Should they become a playable race?
I think this question can be best addressed by quoting a post from an ex-dev who worked on Morrowind and is responsible for a very large amount of lore, including but not limited to the Dwemer:
"A Dwemer of eight can create a golem, but an eight of Dwemer can become one." - Ancient Tales of the Dwemer, Part VI: Chimarvamidium
And that's just taking it from the perspective of understanding our character from a roleplaying standpoint. If we think about a Dwemer going around doing the same tasks and same quests as any other average mortal around Tamriel, it becomes insulting. Part of the fun of quests is being caught off guard, or not understanding something--for example, in the College of Winterhold questline in Skyrim, and we find the Eye of Magnus. While an average mortal will have no idea what the Eye of Magnus is, a Dwemer could most likely figure out its purpose quickly and easily, and, once more, do a bunch of tests on it and harness its power in the most logical and complete way and make more of them and...yeah, do you see where I'm going with this? Playing as a Dwemer should be an entirely different experience than playing as any of the other 10 races. It would require completely custom made scenarios and quests and paths and everything, all just for one race, and that's just to make things not a total disgrace to the Dwemer. I'm sorry, but a playable Dwemer is just out of the question if we're hoping to maintain any semblance of respect for the established lore.
So is that it? No Dwemer interaction from here on out? Not necessarily! Instead, I'd like to stand alongside the rest of you in hoping for some smaller-scale interactions with the Dwemer. There's bound to be more non-hostile ghosts, such as Radac Stungnthumz, whom we could interact with, maybe some that are a little more sociable. Perhaps Yagrum Bagarn is still alive somewhere. Or, maybe there was one or two more Dwemer in an Outer Realm at the time of their disappearance. Many possibilities exist for us to get more information on the Dwemer and learn a bit more about them, while still maintaining the mystery and intrigue around their race. Or, hopefully--better than all of that--we could get to see Numidium again in the Fifth Era, and in addition to learning more about the Brass God, we would, by extension, learn more about the Dwemer.
Even if not, the Dwemer had locations all over Tamriel. There will still be more ruins to explore, more mysteries to solve, and more Dwemer lore to learn!
The potential for the Dwemer is one of the greatest out of all aspects of the lore. Please, let's not waste it.