On the subject of (a) and (b): I recently reviewed the actual rules for Negative Levels, and they're not actually quite as bad as I thought. I was under the impression that, if you gain a Negative Level, that's equivalent in every way to losing a level, and you immediately lose everything you gained at your last level. Not so! A negative level is effectively a template you apply to your character. You lose 5hp, 1ECL, and take a -1 to every attack, save, and check you make. If you're a caster, you lose one of your highest level spell slots. That's it!
Of those things, one is even partially beneficial (it's nice to get experience as if you're a level lower, though things like caster level suffer). You don't need to figure out what you last spent your skill points on or how many hit points you rolled last, you don't lose feats or class abilities (aside from the aforementioned spell slot), you don't lose ability score increases.
At least, not until 24 hours later, when your negative level transforms into level loss if you fail your save, at which point you do have to figure out all those things. But at least you don't have to figure them out while you're busy getting your life force sucked out by undead creatures!
(C) is something of a different story, though. This is one of many places where WotC decided to reduce the risk of actual harm to characters in 4e, and disposed of it entirely. On its own, this is a sound notion. So let's get rid of level adjustment in 3.5e.
But wait! All the undead creatures that deal energy drain are balanced with it in mind! They will be rated as a higher challenge rating than they merit, if we take negative levels out of the equation!
We could simply remove energy drain altogether, and bump everything that had it down by a point of CR. But this has the unfortunate side-effect of making these monsters kind of samey. What's the difference between a wight and a skeleton, if the wight doesn't energy drain?
So what else can we do to replace negative levels?
Consider the Wraith of Stargate: Atlantis, consciously modeled after vampires. They feed on human life force, sucking it out
This winter, I was playing a lot of Demise: Rise of the Ku'tan, and it struck me that Demise incorporates a lot of aging effects. Traps, spells, resting, some monsters, all sorts of things can age a character quickly. Drinking certain potions can restore youth and vitality.
Yet D&D has almost entirely stripped out aging effects in 3.5e. There are rules for aging, but there are precious few ways a character might age, aside from the normal way we're all familiar with. Earlier editions had spells that aged a character, now replaced with experience costs. Perhaps there were good reasons for this. But premature aging is such a prominent trope in fiction that it's a shame to leave it out of D&D.
So let's replace negative levels with premature aging.
One school of thought seems to hold that magical aging should reduce the character's maximum age (the point at which they keel over and die of old age, no resurrection permitted) without increasing their current age. Another holds that magical aging should straightforwardly increase your current age.
The justification for the former is that, for anyone dependent on mental ability scores, advancing through age categories brings bonuses. If your maximum lifespan simply decreases, then you don't reap the advantages of these bonuses. Most compellingly, the intelligence, charisma, and wisdom that you gain are supposed to come through experience, of personally living through those long years.
Certainly, there are some characters who might trade a -6 to physical scores for a +3 to mental scores, but that deal is lopsided for nearly all characters.
As for experience: we already have a system to keep track of that. You know what really comes of experience? Experience points come of experience. Ability score increases come of experience, but only every four levels, and they don't necessarily go into mental abilities.
Most importantly, if your maximum age simply decreases, then you're not aging. If Robert Patrick gets his life sucked out by a Wraith under this model, he simply remains the same for several seconds, then keels over of old age without having aged a day. That's profoundly unsatisfying.
So let's have magical aging just add to your current age. You may keep track of your character's chronological age separately from his physical age (the wrinkled old geezer may firmly insist that he is in fact 23), but that's not strictly necessary. Only physical age is important.
I do dispute the notion that older people are necessarily more intelligent, wise, or charismatic than younger people, but I suppose having nothing but penalties would run into the problem of (c) in the first paragraph. "Oops, my character turned 35, now he's weaker, slower, and less hardy, without anything else to show for it. I guess I should roll up a new one now." Having magically-earned years apply penalties only would fall into this trap, and be undesirable.
So, having decided that energy drain entails adding years to a character's current age, let's consider how to balance those creatures that deal energy drain.
To keep these monsters properly balanced (assuming they're properly balanced in the first place; an assumption some may find laughable, but there it is), they should add enough years per energy drain that it will take as many applications of age to kill the average character as it would take applications of negative levels to kill the character. So we must answer these questions:
1.) How many negative levels does it take to kill the average character?
This one's easy: it takes as many negative levels as the character has levels. Well, okay, so it turns out the real question here is: what level is the average character? Setting aside the fact that Aragorn son of Arathorn, Conan the Barbarian, and any fictional hero you care to name is at most 5th level, I'm inclined to just say the "average" D&D character is level 5, if only because the highest-level campaign I've been involved with was level 10, and because after 10 is where balance between classes completely falls apart.
2.) How many added years does it take to kill the average character?
This one's a little trickier, because it comes in two parts: how old is the average character now, and how old will he be when he dies of old age?
Well, I don't think it's particularly a stretch to suggest that the average character is human. In D&D as in much literature, humans are treated as the baseline, the default, the standard to which all others are compared.
So let's go back to those aging tables. A human of a simple class (barbarian, rogue, or sorcerer), just starting out on his adventuring career, is 15+1d4 years old. A human of a moderate class (bard, fighter, paladin, or ranger) is 15+1d6, and a human of a complex class (cleric, druid, monk, or wizard) is 15+2d6. Which is to say, the average level 1 simple class human is 17, moderate is 18, and complex is 21. In this case, let's take "moderate" as a synonym for "average", so the average level 1 character is 18 years old.
Unfortunately, I'm pretty sure it doesn't say how many years it takes an average person to gain a level, possibly because this is nigh-impossible to quantify. After all, the majority of people go through their whole lives without ever making it past their first level of commoner. So I'm tempted to just say the average level 5 human of a moderate class is the same age as the average level 5 human. Again, 18.
Now, a human dies of old age at 70+2d20 years of age, or 91 years old. So if you age the average human by 91-18=73 years, he dies.
So, we've concluded that the average character is an 18-year-old level 5 human who will die in 73 years. 5 negative levels will kill him, so 5 applications of aging should kill him. Each energy drain should therefore age a character by an average of 73/5=14.6 years. Because this is D&D, that should be randomized.
In descending order, the 5 sets of standard dice that average closest to to 14.6 are 6d4, 4d6, 3d8, 2d12, and 3d10. In general, more than 3 or 4 dice are insufficiently random, and fewer than 2 or 3 dice are potentially too random, so I'm inclined to rule out 6d4 and 4d6 for sure. Because it's slightly harsher than the others, I wound up going with 3d10, but 3d8 and 2d12 would both be solid choices.
As a post-script, I shall attempt to tackle the inevitable objection: what about elves and other long-lived races? Well, yes, what about them?
Under this system, it would indeed take about 29 applications of energy drain to kill the average elf. But that makes sense, doesn't it? An elf has a massive store of life force, that's why he lives for so long in the first place. Of course he's got plenty to spare.
Maybe it's unbalanced, then. An elf is, by his very nature, more resistant to energy drain. But also consider this: what is an elf's favoured class? Wizard (well, in my campaign's world, druid, but never mind that). Wizard is a strongly single-ability-dependent class, which benefits from intelligence and hardly cares about anything else. Up until venerable age, a wizard can only benefit from being prematurely aged. So the very race who is most resistant to energy drain is also the race that could tend to benefit most from energy drain! I call that satisfactory.