I think character hit dice and monster HD work fine for certain type of games. But still I think about other solutions and alternatives. It doesn't necessarily mean I am seeking for better or other way to work things out but am interested in thinking how things could be done otherwise.
I was reading this topic at rpg.net and got few ideas from it. These ideas I write here are more or less my own and mainly influenced by the topic I linked here.
Hit Points Rolled Before Encounter
One idea is, that if character is fully rested he rolls his hit points before the combat encounter. Basically roll HD equal to your level + con modifier x your level. For example 2nd level fighter with con bonus +2 would roll 2d8+4 to determine his hit points. These hit points remain as long as character is fully healed and are rolled after it again.
So if that level 2 fighter rolls 15 hit points and suffers 5 damage he is after that combat in 10 hit points. He doesn't roll his hit points again before he is healed to his original 15. When he is healed, his hit dice are reset and he will roll them again for next encounter.
Advantage in this is that your character is not tied to certain amount of hit points but disadvantage is you never know how many hit points you might have. You might get a burst with a lucky roll and have higher hit points this encounter but next encounter after healed you might roll poorly and have low hit points.
HD As Soaking Damage
This was an interesting idea. Basically you got HD based on your level and when character suffers damage, you roll hit dice + con modifier to soak the damage. For example if character is hit with d6 damage and suffers 4 damage, player first rolls 1HD + con modifier and sees how much of that damage is soaked. If one HD is not enough for that damage, he rolls as long as there is no damage left.
But if after rolling HD soak there is more soaking remaining than damage, what happens? Are those lost or are those left over for next damage?
Example 1. Character is level 4 fighter with constitution modifier of +2. He suffers 7 points of damage. Player rolls d8 + 2 and gets result of 6. 1 damage is left behind and he rolls d8 +2 again for a result of 4. Damage is fully soaked.
Option: But as there was after second HD soak roll only 1 damage and soak result was 4, three damage was not necessary to soak. So 3 points of damage soak was left over. Was it wasted and character has now 2HD soak left, or does that remaining 3 points move to next damage soak?
If remaining soak doesn't move to next hit it means that characters have basically less hit points than just rolling total hit points. Let's do a little math here. Character with 4d8HD +2 con modifier per HD.
1st soak: 6 points of damage, player rolls 1d8+2 = 5. Second soak roll is 4 so damage is soaked.
2nd soak: 5 points of damage, player rolls 1d8+2 for 8. Damage soaked.
3rd soak: 7 points of damage, player rolls 1d8+2 for 9. Damage soaked, no HD left to soak damage any more (character is in 0 state after next damage or negative beyond 0 depending on in what point character dies.
In those three attacks total damage was 18 what character soaked with 4 HD. But if he wouldn't have soaked them, he would have (5+4+8+9=26 HP) 8 hit points left. So basically soaking without benefices of remaining HD results character would have a lot less total HP than if he had rolled his HP in this case.
Character with 1d8HD +2 con modifier would have average of 6,5 hit points per level. In my opinion if damage is soaked the remaining of soaking result should carry on to next attack. Or characters would generally have less hit points than normally would...
1st soak. 6 points of damage soaked with first HD roll 5. Second HD roll gives result of 5 also. There was one point of damage left so character can have +4 from remaining soak roll to next soak.
Rolling dice is part of gaming in my opinion. And rolling dice is fun. I can also see how soaking damage with HD could be fun and exciting but is it necessary? If your soak result doesn't carry on to next attack, it makes characters possibly weaker in hit points. If the remaining soak result is carried to next attack the result is that HD soak is just more complicated way of dealing with damage. Rolling soak could be exciting in combat. You could soak well and shrug off damage, or you could soak poorly and have to burn your HD.
But even if it could be fun in my opinion it just slows down the game and makes it more random.
I could try out this soaking option to try it out but I think it is not a good addition to game. It slows things down, makes things more random and so on. Random is good for hit and damage, but should character's durability also be random? Not in my opinion...
One Step Further - HD Soak + Stable Hit Points
While I am at it, I'll make it more complicated but in a way I could use it (if it wasn't unecessary for rules and over complicated).
From level 2 every second level (4, 6, 8...) character gets soak HD and every other from first level (1, 3, 5...) he rolls his stable hit points. When damage is dealt character rolls soak HD and if there is damage left, it is decreased from his stable hit points. Also if there is soak points left after damage, he gets an additional burst for his stable hit points.
Example: Level 5 fighter without con modifier (to keep things simple) rolled his hit points for first level 5, third level 3 and fifth level 7 for total of 15 hit points. In addition he has 2 HD from levels 2 and 4.
Fighter suffers 4 damage in first round and rolls another of his available HD scoring 3. There is 1 damage left he decreases from his rolled hit points. Second round he suffers 6 damage and rolls his remaining HD scoring 8. He shrugs the damage off and adds 2 temporary hit points (adrenaline surge if you will). Next round he suffers 5 damage but doesn't roll soak HD as there is none remaining. He uses his temporary hit points 2 left from last soak HD roll and takes remaining 3 points of damage from his rolled stable hit points. (If character has con modifier, it is calculated normally to rolled hit points and rolled HD soak!)
This would be the way I'd use it. But it is complicated. You would need maximum hit points (rolled), current hit points (damaged), available maximum soak HD dice and soak HD dice left. Temporary hit points remaining from soak roll you could just add to current hit points.
One Step Further Method - Healing
How to heal HD for soak? If for example normally character would heal 1 HP per 8 hour rest character would heal 1HD per it's average result x 8 hours rest (40 hours per 1d8 if rounded down). For 1d8 HD every 4 points equals 1HD soak die of regenerated hit points via potions or magic. Character would first regenerate his stable hit points and when they are at maximum he starts to regenerate soak points.
For example 2nd level character with 3/6 hit points and no HD would first regenerate 3 hit points and after it his HD.
NPC And Mosters
For NPC and Monsters I would only use HD rolls to keep things more simple for Referee. So with NPC and monsters do what you would usually do. Randomly determine HD by rolling or assigning hit points in the scale of HD results.
These are just ideas based on rpg.net topic. I think these make playing slower and more complicated in addition to needless randomness. But if I would use soak HD I'd use "one step further" method combining rolled hit points and HD soak. It would be both stable rolled hit points and extra variation by soaking with HD. But in my opinion D20 game fighting should be simple and fast. Roll to hit, roll damage, mark damage on hit points. Does it have to be more complicated than that?