Trying to be as objective as possible, these are the first things that pop into my head when I think of what I've seen of vampires in Oblivion and what I'd still want to see. Hopefully this might provide some quest or feature ideas that wouldn't be too hard to implement; I tried to think practically, but I'm not sure how practical it will sound to someone with coding experience.
--Immersion and animations. I loved what I saw in the videos, by the way. Vampirism isn't clean and invisible; it's messy, disturbing, seductive, taboo. It's supposed to be an unsettling experience outside the norms of humanity. A vampire mod to me just doesn't feel very effective without a visible bite--a crouch over sleeping prey, a pounce and grapple upon the neck of a stalked victim, a soft burying of the face in the sweet neck of willing prey. That visual confirmation of what you are is important; mods where you cast a spell to drain health make me feel like a magic user, not a vampire.
There should be a variety of ways to get what you want depending on your nature... to steal it silently, to tear it from the throats of your foes, to stimulate and seduce those who find you attractive, or to accumulate donations from the populace. Each choice should have its consequences--a 'mosquito' should have the easiest time of all, and the seduced or protected should be loyal but possibly possessive and prone to fits of jealousy, while a trail of bodies or an intimidated hamlet should at some point bring the hunters to your door.
--Hunters. Sympathetic hunters, fanatic hunters, grimly respectful hunters... that delicious feeling of hiding in plain sight doesn't really stick with you unless there are those few voices crying out into the stillness, trying futilely to warn the unknowing thousands. To have a dark, clandestine relationship with those who see you, hound you, and attempt to thwart your hunger is essential to feeling that you're a spider in the dark. Your choices as to how to deal with these hunters should matter--if you consistently slaughter, seduce, flee from, or spare your attackers, they should develop opinions about you. The more times you escape their grasp, or murder or confound their fellows, the more notorious you should become. And if they have no friends they've lost to your fangs and claws, they should not be able to muster as much hatred as otherwise. I want to get used to bandits failing to notice my swift-silent steps in the dark, then be surprised by the sudden arrow in my back; to realize organically that I'd become too arrogant in my assumed superiority over humans. I want my hunter to become the hunted as well--more so if it's deserved, and less so if it's not.
--Dark choices. I want to feel that I always have a
choice, the possibility to be cruel, neutral, or kind... and I want it to be noticed somehow. If I kill non-violent creatures wantonly, I want to be hunted harder, hated and reviled. If I never kill anything that doesn't attack me first, and use my powers to protect the innocent from my own kind, I want even the hunters to be reluctant to take me down. And this could perhaps change my appearance and abilities; if I kill constantly I should become better at it, and if I try to heal and protect and save, maybe my physiology will change. Perhaps the sun and the gods will despise me less; perhaps my bite will numb pain instead of causing it. But if I refuse to feed at all, this should lead to the hunger mastering me instead of the other way around. Moments of mercy are key, to emphasize the darkness around you. It should always be the player's choice whether to be a monster, a dark angel, or something in between, but if that choice is to mean something then it must be rewarded; you must become more vicious, dangerous, and unholy or more controlled and less reviled. The fame rating could change based on how often you free cattle/kill your own kind/spare hunters/heal someone diseased or left to die by bandits using your hard-won blood, perhaps; you could be the cause of rumors that a dark protector roams the streets at night. And with enough distance between fame and infamy, perhaps a true monster's bite will cause enough pain (or even poison) that feeding on sleeping victims is no longer possible, while a gentle monster's bite will not do any extra damage to enemies (or will paralyze briefly?)
Emphasizing the costs and benefits of being unholy vs. torturously fighting to retain your humanity and balance your predatory needs is essential to the experience. Same with being a seductive and manipulative predator vs. being a creature of base cruelty, enjoying the fear of others. This is very immersive, seeing the effects of your choices transforming what you are.
--Cost. I don't want neverending blood banks as I've seen in some mods, I want weak and fragile mortals who can lose about the same amount of blood as a human (two pints) before dying or needing healing. That's part of the tragedy, part of what made the Vampire: the Masquerade pen and paper such a good roleplaying system--the fact that you may not have
meant to kill that person, but you did... and that's how the loss of your human side begins, when you realize how easily you leave destruction in your wake regardless of your intentions. When you can't get that first life back, you weep bloody tears... but what about the life after that? The one after that? And on and on... It's so easy to begin to live as if they're ants, and it's no tragedy to crush something so small and insignificant between your jaws. It's the law of nature, the law of beast and prey. It's so much easier to just drain one body wherever you find it than to carefully take small amounts from the young and strong and healthy. Power and strength carry a blood price.
--Storytelling. It should always feel like you're living in a fluid world... things are changing, moving as you act and interact. There are always eyes out there, watching you from the darkness. If you don't conform to their will, perhaps you should be forced to actively avoid those more powerful than you.
--Dark romance. Sensuality and sensation are so much a part of the vampire myth. If you feel your humanity slipping away, at times you will use what you have left of it to grasp what you can. You will reach out to the six you're attracted to in a lost attempt to clutch and to save. And there should be dimensions, choices... will you gather a male, female, or mixed-gender harem of slaves? Would you wish to be adored by a new mortal each night, each week, or to gather a crowd of jealous lovers to fight--with words, deeds, swords, or magic--to win the right to be yours this evening? Or will you choose one beautiful, caring mortal (who is either immune to your powers or whom you spare from your mind control) to share your secret and sacrifice his or her blood to your hunger? The temptation to take whatever you want simply because you
can is immense. Do you still respect the freedoms of others, or will you grow jaded and leave a trail of broken hearts and bloodied bodies? And even if you mean well, can you resist the temptation to turn your lover instead of watching them die of old age? If you do resist, what will happen if your enemies discover your powerless mortal beloved?
The situation grows even more interesting, perhaps, if you choose to play Romeo and Juliet with a hunter, each of you transgressing the boundaries of your respective worlds, trying to overcome your wary mistrust of the other. Perhaps this stalking game finally culminates when you can no longer resist one another--or when your hunter can no longer live with allowing you to live, forcing you to kill them.
Well, in closing, I'm very much looking forward to this mod, and if you're ever looking for female voices I'd probably audition. Though you're right that it's impossible to please everyone, based on the features I've read about and seen in the videos I think this mod will suit my preferences very well, and that it is extremely faithful to the classic vampiric feel, which will add a whole new dimension to the game. I was ultimately quite disappointed by the Terran Vampires mod because it felt
too much like VtM, and not just VtM but a bit of a copy of the CRPG as well. I had hoped for unique new characters and a streamlined-for-Oblivion system as well as a dark and rich atmosphere, but that mod was too focused on politics and cool powers. There were so many clans that it felt almost pointless to choose, and you couldn't exactly say there was a rich tapestry of quests and interactions. Past the initial quest, it fell pretty flat for me. I just didn't really enjoy continuing. I was thus delighted when I heard about this mod.
I like the idea of the rogues being normal, personally; one of the best ideas of VtM was always that the true horror is not what the vampire becomes, but what the vampire
is--[b]human.[/i] Despite the monster that you are, you always had a choice, but you sink deeper and deeper into rationalizations, telling yourself that you're better than a human when in fact you are becoming more like a frighteningly intelligent but utterly rabid animal. The fact that the player might start out feeling sympathetic, being careful, and then end up just draining carelessly due to convenience is an extra layer of connection with your vampire--you feel the temptation to lose your compassion, to see it as merely a vessel. I think it would actually be interesting if you start out giving the player the Blade II "they're a different kind of creature than we are, it's safe to be a covenant vampire" impression, then pull the twist that the drugs they take that "slowly transform" them and consume their humanity are placebos which have no real effect on vampires--their condition is actually a result of releasing the predator within, and every covenant vampire could become as monstrous if they didn't adhere to a code! Realizing that if you didn't direct your hunger you could become exactly as hideous what you've been fighting is, I think, a good way of adding an extra layer of horror to what you already are. Here you had thought you were higher than these monsters, but if let go of your self-control--embrace the chaotic, mindless, predatory instinct and allow it to consume your free will--then you would be no different than any of them. (To become a rogue, you could acquire a new power called "embrace the monster" where you black out and wake up with a pile of corpses around you; you let the predator take over and gorged yourself far past simple nourishment and you are beginning to change.)
Very glad you're working on this, and thank you.

One quick question--will the feeding through dialogue feature an animation? Like, after you exit dialogue, you pull the person to you and press your face into their neck in first and/or third person?
Also, an idea--telepathy could allow for voice without lip-synching. Maybe some vampires are able to speak to you directly; a small effect placed on the file with an audio program could simulate a mind-voice, or even just acting. I'm not sure if this would actually save you some work, but just in case, I thought I'd mention it.