I would hope that they just stay with the infected by a werebear lore than, and make it traditional forced transformations and such.
I would like encouraged feeding for vampires, but I'm not too sure that the Weaknesses should get nerfed as you get more powerful.
I liked Oblivion's vampires, they felt like something you really didn't want to be. but of course we're going to need at least some reason for a few people to be one.
The problem was that the Bonuses were never actually worth the given weaknesses, So just make the bonuses by some views worth it.
For weaknesses, not much changed from Oblivion. as a new-blood, Sun-damage isn't really a worry, but it's still present. the casual Frost resistance & Fire weakness. as going without feeding, just like we've had, these symptoms become more potent. But for feeding, Instead of being reset exactly back to how you first started, your weaknesses are a tiny bit, maybe almost unnoticeably a bit more potent, however your Strengths are a bit more "noticeably" potent.
by this, even though you're feeding brings you down in the symptoms, you're still generally better than how you started, and this could add on as you continue this.
For the stages, in this case they shouldn't be random huge jumps in difference, and instead of the last stage you can reach at that point being the strongest Vampire you could be, howbout we limit it to just something decently better than what you are now, and can't get any higher than that at that point. of course, by doing this, after you feed and you're a little better than you were before as stage 1, the newer last stage would still be "something decently better" than stage 1, in which your new stage 1 is better, making your new Last stage also better.
this long slow process of Feeding & not feeding would still increase your weaknesses, but your strengths increase faster than them. (This could go far, but there would eventually have to be a limit where it does just repeat itself from feeding and your possible last stage)
For what the strengths are, I really did not like any of them in either Oblivion or Skyrim, I'd prefer a less "Inherited magic" route. leave Vampiric Drain as a spell to be learned from a Vampire faction, from a Mage there or something. Let's have the character slowly become more like a predator.
a good example: What if a high staged powerful vampire was quite similar to how the Predator thingy was like in Far Cry: Instincts? Sprint at pretty high speeds, jump like a boss, smash people pretty hard with your weapons or even just your fists, follow scented trails of living things to stalk them down. Something like this could be what your character would be progressing to. let the other things that are more based to classes be earned from Vampire factions and your choices in them.
Some could view these things as a bit Over-powered, but let's not forget you're now forcefully Nocturnal (unless of course you want to die by daylight), and basically incinerate at some contact with fire. (being forcefully nocturnal came well with another Idea I had, that in the next TES, Nighttime could be more feared, where higher level, or just potentially harder creatures turn up only during then. Being an Over-powered Vampire would be suited for it)
Some other fallback features to add to it--
Another reason to encourage feeding, it could also give a timed bonus of strength, or just leveling speed like the Rested bonuses do for sleeping in Skyrim.
Wearing apparel that covers more of your body could reduce sun-damage, maybe even full blown protection could stop it. but as a negative affect, you might rise suspicion for NPC's if you're going around wearing a complete set of robes that covers your face to your toes.
That's all I've got, sorry if I'm a bit unclear on a few things. my Idea's don't feel well organized or worded.
My problem with Werewolves is that the combat gets boring. Mainly because there's no real skill in it - mash the mouse buttons until everything dies. Now, the bigger problem is that this is my criticism for melee combat in general (werewolf combat is just a bit more bearable), so it'd be a bit harder to fix within the normal system.
I haven't played enough with Vampire Lords to really give any criticism about it.