WEAPONS:
Weapons need to be most noted as they have mechanics such as critical hits that need to be modified for this sortof houserule. Actualy it's not so much criticals that need modifying (20=you hit them), so much as crits (triggering the need for a confirmation roll) and confirmations. Confirmations will now permit you to deal an alternate set of damage if you so choose, ussualy better. But you can't use both. (Unless otherwise noted) Most often you'd need a confirmation to deal mental stat damage. Think of it as a headshot. Here's an example.
Spear
Fortitude Save DC: 13+strength modifier of weilder
Innitial Damage: 0
Secondary Damage: 3d6 constitution
Crit: 19-20
Confirmation Innitial Damage: 1d4 strength and an additional 1d8 strength if the wielder of the spear has a strentgh score 15 or higher.
Confirmation Secondary Damage: 0
This is just an example. Clearly this isn't exactly well thought out. Let alone balanced. It's just what a weapon might look like. There is ofcourse going to be strong emphasis not only on physical stats being hurt, but on being used to make weapons good. Makes the DCs higher, and the weapons more powerful.
SPELLS:
Spells function as weapons, though they probably will not be nearly as impressive. And when they use attack rolls they'll be a bit more MAD then usual. Here's a taste of that medicine...
Lesser Orb of Acid (remember, ranged touch attack)
Fortitude DC: 9+key casting stat modifier
Innitial Damage: 1d3 charisma
Secondary damage: 0
Crit: 20
Confirmation Innitial Damage: 1d4 charisma
Confirmation Secondary Damage: 0
I let this one deal charisma damage cause even though acid is very painful, it's main thing is scarring you right the **** up! This is only slightly weaker then a weapon might be. And cause of that, as the spells progress, they will tend to deal more HP damage. Yes, HP still exists...
HP:
HP still exists, it's just greatly reduced. That way there's a diverse array of ways to kill the foe. (Or help them survive if they're dealing damage in different ways) They still get the bonus HP for a high constitution, but it hit die wise it's like
HD d4=1
HD d6=2
HD d8=3
HD d10=4
HD d12=5
Albeit in a game like this, every little bit counts. However, by level 20 you'll accumulate probably enough HP to survive more then your fair share of rounds. And sense this is based on encounter tables instead of daily tables, you probably have nothing to worry about. A level 20 barbarian with 18 constitution has 180 hp. (Not counting anything that might increase it further) Even the more harsher spells won't be immideatly killing him too quickly... It'd be more practical to try to get lucky with a weapon.
More monsters will have weaknesses to try and help the players. More emphasis on those weapon statistics like wood, iron, silver, etc. More emphasis on elements. Immagine how lesser orb of acid will deal 1d3 x2 or more damage against a troll! +X weapons are not effected much either, as it's safe to assume whatever damage they deal is increased by +X, and +X on attack rolls.
POISON/DISEASE:
This use to be primarily how poisons/disease made themselves known. I've decided poison/disease will be much more blunt, and considering the many effects that be done, it's pretty simple to comeup with them to do things without resorting to damage. Poison however, can get tricky, and may lead to some very difference prices because some will be downright impossible to live through while others will just be really annoying.
Blue Ring Octopus Poison
Fortitude DC: 13
Innitial Damage: Stunned for one round
Secondary Damage: Under the suffocation ailment for ten minutes.
Rattle Snake Venom
Fortitude DC: 15
Innitial Damage: Make a fortitude save DC 10 three times. If you fail 2 out of 3 of those saves, your DM chooses a limb that disintegrates.
Off-hand (you cannot use two-handed weapons and suffer some chance of spell-failure)
Major-hand (like off-hand but worse)
A leg (move at half base speed and have to make a reflex save or fall prone untill you get a prostetic)
Secondary Damage: 0
Bubonic Plague
Incubation Period: 5 days
Fortitude DC: 22
Damage: 4% chance on d100 percentile chance of dying.
As you can see, poison and disease become alot more serious buisness with more drastic and permanent effects. This will make people emphasize more on stratagy and forethought then blindly going into combat. Non-casters might actually get to see a little more action, now that they have presumably more advantages.. Rogues will need perhaps the most serious thought of how to handle them. Probably something simple like +1 damage per what you would've otherwise done in +1d6 in whatever sortof damage you were dealing.
As I said, this is all just a thought. I havn't put this into practice and have no idea of the longterm effects if such a game was ran. It might lead to player's demises too quickly, it might make them feel like gods if they can get surprise rounds or win innitiative. Either way I think it'll be alot of fun to atleast try for a short-game.
But won't every successful hit with a weapon cause you to have to re-calculate your fort save, ref save, attack bonus, carrying capacity (suddenly you become heavily encumbered), skill bonuses . . . sounds like a lot of work?
Reminds me of Traveller :D FGMP 15
