You have vampires that can turn into mist, and men/mer who can transform into werewolves/boars/bears/sharks/crocs/vultures. Is it really much of a stretch for there to be a bloodline that can turn into a nightmarish bat thing? You ask me, there isn't. As been said by many, the lore on vampires is so vague and there is so little of it, seeing this is not surprising in the least of bit. Also, the vampire lore is so sorely lacking and just not there, I whole heartily welcome my new batmen overlords.
Your right, it's not a "stretch". There are many things that could just be thrown in that wouldn't be considered a "stretch". Two headed Dragons, Greek Gods, Wookies, you see where I'm going with this? It wouldn't be a "stretch" to add demonic possession to Harry Potter, but you see how that affects the story? The world? Story and consistency are important. Without them there is chaos/confusion and then writing becomes redundant because what is the point? Might as well hire a 5 year old to complete the story at that point. And there is actually plenty of lore and precedent to create the Volkihar and other Vampires of Skyrim. There was no need to grasp at ideas from other sources.
Believe it or not people, Bethesda CAN add new lore. Lore isn't just thrown into the world one day and then never changed again - its constantly being added to, changed and altered.
If you think that Bethesda are just going to throw random things into the game without any lore, then you clearly don't know much about TES. There are plenty of logical explanations as to how the Vampire Lords came about, and why we've never heard of them. And since we know next to nothing about the Castles, it's ridiculous to assume that they're just going to be thrown in beside a city.
My advice? Don't decide you hate something until you have all the facts. Try WAITING until you've played through the quests before complaining about how Bethesda have made the worst game ever.
You have a point, the facts are unclear at this stage. Your earlier statement about adding new lore is correct but in this case TES has been around for a while and I would argue that we have enough information on Vampires to have an idea of what to expect from them. Never have they had such dramatic physical changes and never have they been associated with bats. That did come out of nowhere, considering that we have known about the most powerful Vampire clan in Skyrim, the Volkihar, for years. Lore shouldn't be constantly changed or altered. That is what we call RETCON. No one who cares the least bit about writing or story should accept RETCON.
Everything I've seen in Dawnguard Trailer is lore friendly.
Vampire Lords = Ancient Vampires.
The Vampires in Dawnguard will most likely be Volihar (Spellcheck?)
Their Fortress is prolly the Icy Fortress buried in snow. (Went there in Arena; anyone else remember?)
And the Dawnguard are just Vampire Hunters, really. So they can easily be added. I see no Lore-Breakers in Dawnguard Add-on.
We have read about and encountered Ancient Vampires before. They had no wings. Am I to believe that they just suddenly grew them? I am unconcerned with the fortress. They can't fit every building and town we have ever heard about in the game. That is reasonable gameplay/story segregation.
We retcon our own world all the time as new discoveries are made. Having things change just makes The Elder Scrolls as a universe more alive.
We UPDATE our histories with new discoveries. Unlike in a long running video game series, we do not have the benefit of actually and collectively witnessing myth becoming real. We do not have access to all the information that has been lost or spread throughout the years. We can go from one game and create history and then to another to witness the aftermath hundreds of years later while retaining the accurate info that would otherwise be lost and altered due to incident or lax of memory. We may never know what REALLY happened to the Mayans or the Dinosaurs or what have you, but in a video game we can just go load a previous save. We cannot RETCON anything in the real physical reality; changing the past is impossible but we can manipulate history books and information. We are not going to wake up tomorrow and find Elves fully integrated into our society.