The lore is there, and it meant to be twisted and expanded.For me personally, I don't care if putting Vampires or Werewolves will break or whatever the lore.I want to see something bad ass, and I'm tired of watching all those YouTube videos turning TES (in the face of Oblivion) into a mockery.I would love to play either as a Vampire or Werewolf, as long as there are enough quests to keep me occupied, and there are factions to interact with.It would be cool to become a Vampire and hunt Werewolves for example, and so on.More factions/content is instant win as long as it's bad ass/epic.(For example look at the older Elder Scrolls games, and note how many factions they had, and for how long the player would have been occupied doing their quests) It's simple as that.
Putting vampires, werewolves, even werebears in the game won't break the lore at all, all of them fit the lore perfectly actually, werewolves are found in all parts of Tamriel, Werebears are the most common lycanthrope in Skyrim, and there's a book that mentions a particular clan of vampires in Skyrim called the Volkihar.
If they aren't all added most of the community will be pretty upset.