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They already have been, so yes. Vampires have been in each game. Since Daggerfall for sure, haven't played any before that. And werewolves were my favorite bit about Solstheim. I absolutely loved being one. I can't see any reason why they couldn't work.

Sure it can be pulled off. There is always room for improvment but I think we will see both in Skyrim.

Of wollllf and maaaaaaaaaaaan! Yeah!

Yep vampires are easy. Oblivion made progress by having the character look different and allowing you to actually feed. All that needs to be done if work out a biting animation so You can latch onto enemies and kill them by draining thier blood and bring back the vampire clans or some kind of vampire factionfrom morrowind. Simple

Werewolves were handled well in bloodmoon and it was fun to be one so there doesnt really need to be any gameplay tweaks.