Before I get into energy drain, I want to say that I like the flavor material for the wight. It seems to rely heavily on 2nd Edition's Habitat/Society and Ecology entries, specifically the bit about animals fleeing from it and plant-life dying around its lair (both of which are absent from later editions). These could be used to provide clues, as well as just add some description.
I am also a fan of the wight being someone who was cursed (or blessed, depending on who you ask), as well as wights being able to potentially retain abilities that they previously had while alive. It makes more sense and adds more variety (why it was not a template in 3rd Edition, I have no idea), which can allow for more compelling victims.
In 2nd Edition energy drain was very dangerous, as far as I can remember involving level loss on a per-hit basis. This meant that if you went toe to toe with a wight, wraith, or many other forms of undead (and probably other living monsters) you could find yourself in a death spiral as you became easier and easier to kill, while they became harder to hit. If you were a spellcaster, this could involve losing access to your best spells.
Thankfully, these lost levels could be restored through some forms of magic...if you or someone you knew had access to it. The one time I remember running into a wraith, the DM was kind enough to at least provide a restoration scroll in a prior treasure hoard, so it was fortunate that only one of us--the cleric, of all people--got hit before we were able to drive it off.
3rd Edition made things a bit easier by instead having each attack pile on negative levels, which you could make a saving throw--against each--to shrug off after the fact. If you failed, they could still be removed via magic. Not much, but it was a free chance at least, and because it was always a Fortitude save you could slap on a buff or two in order to hedge the odds in your favor.
4th Edition did away with level loss entirely, instead using temporary conditions like weakened (half damage) and loss of healing surges to represent your life force being sapped away. Some people liked this, others...not so much.
Another common theme of level draining monsters was creating spawn; if the monster killed you, you came back as an identical, or at least somewhat similar version of it depending on your edition of choice. Each edition has retained this in some fashion, though oddly 4th Edition's wight lacks it.
The major problem with creating spawn is the simplicity and speed of the process. If a wight or wraith can create another wight or wraith by killing someone, and they are driven to kill, then what is stopping them from amassing an army? Though the duration varies by edition it is not a lengthy process, and it is not like the original has to give it instructions on how to be an undead monstrosity that hungers for the souls of the living. It would be like eating a sandwich, only to poop out a fully trained, loyal soldier (that can also eat sandwiches and poop soldiers).
|Though given how fast it works, maybe this|
analogy is better served by another kind of "food".
I do not think that just killing a creature should create spawn. This is where Wyatt's idea of having to actually use a kind of soul-draining ability on a creature that was just dropped (or dying). It takes time, heals the wight and since it devours the creatures soul allows it to control the corpse. So, something like a zombie, but not "just" a zombie. Something with a semblance of intelligence, like the geth from Mass Effect or the immortial legion from Fullmetal Alchemist: Brotherhood. Killing the wight could free the souls, with the added effect of stopping these spawned undead (making killing a wight a nice plot hook).
This makes it a bit harder to give wraiths their own schtick, unless you want to stick with wraiths being wights turned up to 11 (or the leftovers). The description Wyatt provides clashes with the origin presented last week; rather than ghosts bound to a spot, they are "pure evil concentrated into a single incorporeal form", which sounds better to me.
Of course, do we need to give every energy-draining critter its own mechanic? Maybe wraiths also cap hit points, but some other method is needed to restore them? Maybe you have to use magic, or destroy the wraith that "claimed" them? I do not see wraiths as the type to have dreams of conquest, but more like ambush predators that lie in wait for living creatures to intrude on their territory. Like ghosts, they cannot really leave the area where their remains are interred (and so cannot freely roam the country murder-poking everyone).
Of course this does not mean that a wraith cannot plot to rule over the living or be encountered anywhere not a tomb/catacomb/crypt/graveyard/cliche-evil-location; the original person might have died in the city limits, had their remains transferred, a city might have been built atop a ruin, or it might be attached to an object stolen from it.
As for spawn? I am thinking not. If a wraith is consuming bits of someone's life force, I can see it empowering them, perhaps with an attack bonus, damage bonus, hit points, healing, etc. For a more complicated wraith, what about being able to draw upon that persons skills and memories? A wraith that strips away a wizard's soul might be able to cast some of the spells she knows (or knew).