I first got introduced to the "vampire" feature in morrowind. I didn't even know what it was, I was kinda suspecting it given the name of the disease, but I didn't expect the game to actually turn my character into a vampire. I was pleasantly surprised when that happened. Loved it for a few moments too. Until I noticed I can no longer continue the game. So I had to reload the game leaving this feature aside for the time being. After a while I came back to it, but it seemed rather hollow, given that even if the game turned you into a creature of the night the gameplay did not reflect that transformation. There was no uniqueness, no specifics. You were just a guy who couldn't go out during the day, nothing else. Fortunately, you did find other vampire clans that you could hang out with and a few quests here and there. Overall, at the time, it seemed almost enough.
Now, Oblivion takes another approach, this time vampires can enter society given some requirements are met. I liked that. I liked that you had to feed in order to do something, in this case, look human(or whatever is accepted for human), it added to the actual experience, you know, having a reason to venture outside and look for prey. Your prey were sleeping humanoids. Again, I was pleasantly surprised by the steps you had taken to improve vampire game play. Sadly after a while I realized that the only difference between the oblivion vampire and the common mosquito, was size. The game play felt the same, the vampire abilities were not unique, but just common spells that could be learned by any character. Combat still very much the same, feeding being only restricted to sleeping npcs.
I don't know if you plan to introduce vampires in Skyrim. I assume you will, looking at the past two games. If you do, I ask you, please, give them more attention. Allow players who choose this type of game play feel a difference other that having to stay indoors at certain times. Allow vampire characters specific combat moves like grabbing someone and tearing out it's throat(since i saw you can actually grab enemies in game). Give them unique spells, I'm not sure what your exact idea of a vampire is for your game, but transformations, charming, even dialogue lines. Something that reminds you why you chose this.
As for werewolves, I can't really say anything about them since I enjoyed the change. It was simple, yes, but it felt appropriate. Even if the transformation happened a bit to often for my taste, it had that right feel to it - go out and kill to quench the beast's hunger or rage, maybe both. Whichever the case, if was interesting. I hope you will consider bringing it back, seeing how it did not appear in Oblivion.
A few suggestions...
Well I can't really be any more specific than the modders who made some wonderful vampire-related mods, within their limits. So I guess the things that appear there are what the majority of players who get their kicks out of playing vampires, want. More quests aside curing yourself from the affliction. Perhaps even a guild style chain of quests.
A more strong reaction from NPCs. And I'm not just talking about their reaction to seeing the actual vampire, you know " Shoo creature of the night!". I'm talking about adding a few more lines to that "rumors" option you will probably have, like in previous games, that talk about a vampire or werewolf attack. Another reaction would be vampire hunters appearing in places vampires were seen, places they attacked. Little things like these that acknowledge your choice to play as a vampire or werewolf.
Like I said, there are mods out there that voice this ideas, suggestions, whatever, better than I ever could.
I hope you'll read this, furthermore I hope you'll take action.