Heh, actually constitution drain sounds interesting too. How would you do. 1d4 CON per strike maybe....ouch.
Oh, I would mirror AD&D with Vampires draining 2 CON, others just 1 CON or something instead of a variable, but really anything could work.
It is a bit of a bookkeeping hassle if you
actually adjust CON with Max HP dropping, etc. but not too bad I would think.
Wow - a level of exhaustion after recovering from 0hp is a really good idea
Yep, this is a pretty common house-rule to help prevent whack-a-mole (and it works IME).
The only argument against it is the death spiral effect, which a lot of groups don't want.
Never forget that you can target a character's Proficiency bonus
Which is also a good measure if you want to keep the number of hits potentially more lethal. Keep in mind the different options:
Exhaustion - 6 to kill at any level.
Proficiency bonus - only 2 to 6, depending on level.
HD - only 1 (yikes!) but up to 20 (wow!), depending on level.
CON - minimum 8(ish), but up to 20 at any level.
Max HP - anything from 6 or so up to hundreds, depending on level.
So, first question is do you want it level dependent? Then I would avoid exhaustion for the most part, unless you allow a save, in which case level and proficiency can become a factor in making the save.