(I just wanted to get that off my chest before I typed anything else.)
So here are my ideas for my very own ruleset. I have typed up some design goals. I want my ruleset to be:
Compatible. There are a lot of excellent modules out there, from TSR to OSR. I want to be able to use those without any conversion. It should also be modular enough that other people's houserules/retroclones can be applied with minimal/no tweaking.
Self-limiting. I want diminishing returns in all things. Other editions have had HP, attack bonuses, and damage that could scale indefinitely. I want diminishing returns (and I even wrote an article about it). HP should start to taper off early. Then skill increases should begin to taper off. Then attacks. I want this because I want level 1 characters to stand beside level 9 characters and still be able to contribute, and I want level 8 characters to still fear 1 HD goblins.
Minimal. Whenever possible, consolidate 2 rolls into 1 roll. Whenever possible, adjudicate 1 roll into no rolls at all. Complicated tables can often be simplified into progressions, which can often be simplified into formulas. Spend less time fiddling with modifiers and rules, so that more time can be spent playing.
Simple. Rules should be easy to learn and use. Steep learning curves are one of the biggest challenges facing tabletop RPGs, since they are such a huge barrier for potential players. A simple game is easier it becomes to introduce it to new players and/or suggest it on family board game night (and even simple rules can offer tactical complexity and meaningful choices). The game can only happen once the rules get out of the way. And I say that as a person who fucking loves thinking about mechanics.
I typed up a character sheet.
Take a look at it, so I can start walking you through the ways it's different from B/X D&D. I'll go right down the list.
Level is pretty much as you know it. It is divorced from HD, though. Level 1 characters have 1d6 HD. Level 2 = 2 HD. Level 3 = 3 HD. But after that, you only get +1 hp per level. Level 4 = HD 3+1, etc.
HP is rerolled every level, and discarded if it is lower than current HP. You might not think this makes much of a difference, but it gives some beautiful average curves.
With the right ability score to ability modifier table AND capping the HD at 3, there's a very nice moment at level three where you can stop saying "add your Con bonus to every HD" and start saying "every point of Con higher than 10 adds to your HP, every point of Con below 10 subtracts from it" and it is exactly the same thing. It just seems more granular, you know? Every point of Con matters more, not just the ability bonus break points. Every point of Con corresponds to +1 HP. (This doesn't work if you use 3d6 for ability scores, only 4d4. Weird, huh?)
Templates revolve around the idea that races and classes are just templates that are applied to the Basic Adventurer. You can't be a Basic Adventurer (unless you're doing some level 0 bullshit), but it's what everyone is built from. The Basic Adventurer gets 1d6 HP per HD, a +1 increase to attack every 2 levels (up to +4), and a save that increases by 1 point every level up to a maximum, similar to S&W. And then templates just build off of that, by adding or subtracting stuff.
You get 2 templates at level 1, up to a maximum of 4 templates at level 3, at which point your PC is considered "full grown" and "totally badass". Templates are measured as 25, 50, etc, which you can think of as percents. If you see "Fighter 100", it means that the character is 100% fighter (and is at least level 3). A 1st level fighter will read "Fighter 50".
I figure this choose-4-templates-as-you-level up thing will (a) give players a chance to "multiclass" from level one (if they want to be a Fighter 25 / Cleric 25), (b) make multiclassing (and monoclassing) dirt simple, and (c) some character concepts are easily expressed via multiclassing, so you could make a Thief 50 / Magic-User 50 and call it a Bard, and give it an appropriate spell-list. Easy.
AC is ascending, has a base of 10 and a soft (non-magical) cap at 18, like god intended.
Injuries are cool. Instead of dying at 0 HP, I just roll on the Big Table of Fun Ways To Probably Not Die (using subzero damage as a modifier) to see if you lose a hand or begin bleeding out. Also on this table: instant death.
Save is also ascending and uses a roll-under mechanic, like making a stat check. It's calculated from 5+level+Charisma bonus, because I figure Charisma also a measure of how much heroic chutzpah you have, and how much destiny wants to keep you alive. But more on this later.
Ability Scores are rolled using 4d4. Compared to 3d6, the average is 0.5 lower, and the spread is a bit narrower. I think this is a good thing, because I intend to use a lot of roll-under ability checks, and an average success rate of 50% is marvelous, and important enough that I don't want to see any characters with 17s or 18s at level 1 (since 85% and 90% success rates seem excessive, don't they?).
I also think that ability checks should be used more, and saves a bit less, perhaps. Use Con checks for drinking contests, use Int for seeing through illusions, those sorts of things. Save the saves for important things, life-or-death-or-dragonbreath things.
Movement is something you've already seen. It's based on race. Humans have a Movement of 12+Dex, which is pretty analogous to the 12' you've seen listed as the base speed elsewhere.
Initiative is equal to your Wisdom score. Whenever you are in combat and try to do something before a bad guy, try to roll under your Initiative. Can you cast a spell before the orc gives you the chop? Can you jump off the boat before the kraken grabs you? I am using Wisdom instead of Dexterity because (a) I think Dexterity is an overloaded stat, and (b) acting before your opponent in a fight isn't necessarily about how fast you can swing your arm, but rather how fast you can think, react, anticipate, predict. And that's the advantage the fighter with high Wisdom has.
Stealth is equal to half of your Dexterity score. You try to roll-under it whenever you want to sneak up on someone. I like this because I think that people will be more tempted to try sneaking around if they have it written on their sheet, instead of it being the expected purview of the Thief.
EVERYTHING USES ROLL-UNDER (except attack rolls and opposed stat checks, probably)
Saves, ability checks, movement, initiative, stealth, even AC can be used as a roll-under check. Bonuses can be applied universally (Oh, you're cursed? That's a -4 to all of your roll-under checks for today).
You're probably already familiar with using ability checks to figure out if the PC can do some basic action. But have you ever tried using these phrases when you're DMing?
"Roll under your Movement to see if you can get through the porticullis in time."
"So you're firing your arrow? Roll under your Initiative to see if you can fire an arrow in the charging orcs eye before he reaches you."
"The great rosebeast opens it's bloom and sprays a cone of thorns at you. Roll under your AC to see if you are hit."
(If you are raising your eyebrow at this last one, consider that asking someone to roll-under-or-equal to their AC to avoid an attack has the exact same odds as an anemic goblin attacking them with a -1 to his d20 roll. If you say "roll under your AC with a -4 penalty", that's the exact same thing as a giant badger attacking with a +3 bonus. Just think about the pros and cons and exoticism of this for a minute, okay?)
And the other great thing about using a lot of roll-under mechanics is that players can immediately look at their character sheet and calculate their odds of success. If they see Stealth 9, then they know that they have a 45% chance to sneak up on the bastard. Save 12 = 60% chance of success.
Compare to that +3 on your attack roll versus an AC of 15, what are the odds there?
Need to roll a 12 to hit, so that's 7-8-9 numbers on the die that will hit, so that. . . 45% chance of success.
Using roll-unders lets players see at a glance their odds, which helps them evaluate the situation and better informed decisions.
Inventory is measured in Inventory Slots. You have a number of Inventory Slots equal to your Strength score, and a single Inventory Slot can hold something like a sword or three days rations. You can exceed your Inventory Slots, but every item in excess gives you -1 to Movement. The number of items that you can immediately access is limited by half of your Dexterity score. I wrote a whole post about this here.
There are a few caveats, like armor takes up a number of slots equal to its AC bonus. So a wizard with 6 Str technically can put on that full plate (+6 AC), but anything beyond that (even a spellbook) is going to begin slowing him down tremendously.
Skills are measured in Skill Slots. You have a number of Skill Slots equal to your Intelligence score. Skill slots can hold more than just skills. They can also hold the bonus languages you know, any weird techniques you've learned in strange places (like Kung Fu), or spells outside of your school (if you are a wizard). Wizards who have 17 Int can pick up plenty of damn skills, languages, and exotic spells, but the barbarian with 5 Int will have to be a lot more choosy. Does the barbarian want to learn how to speak Orcish, how to sail a ship, or how to throw his axe 100' with melee-level accuracy?
There is a strict philosophy of "Skills should not be useful in combat. Use ability checks for that." I don't want people choosing skills based on what they think will be most useful. Climbing walls, stabilizing a dying comrade, sneaking up on a hill giant, etc, should all be ability score (or Stealth) checks. Skills are by definition things that aren't directly combat-useful, so, because of their diminished importance, players will (hopefully) be encouraged to take skills like Dancing and Carpentry. (Getting two random skills at level 1 will also help this, I hope.)
There is no skill list (although I still bet "Pick Lock" and "Forest Survival" will be popular choices). There are no social skills. Use roleplaying and Charisma checks for those. (You don't have to talk in a funny voice, just tell the DM what sort of things you say to the dragon.) It's trivially easy to pick up a new skill at Rank 1, just attempt it 3 times over the course of an adventure and then write it down when you get back to town/finish the adventure.
Skills are rated on a d20-roll-under system, like nearly everything else. Marvelous. It's actually a refined version of what I wrote down here.
This will be controversial: skills are not tied to level. Skills are limited by level, in the sense that you cannot raise any skills to the maximum level until you are at least level 6. Instead, you raise skills by using them at least three times, then you have a chance to improve them during downtime (similar to how BRP does skills). The chance is diminishing, so you will have to be very lucky to max out a skill, even at level 6. You might not be able to max it out until level 12 or something. Maybe you'll max out Dancing before you max out Surgery. Ha, just like real life.
And because there's no limit to how many skills you can test (and raise) per level, there is an incentive to use all of your skills all of the time. I know you grognards are used to finding ways to extract a tactical advantage from commonplace things (like buckets of lard), but I think this sort of thing will really help neophytes pick up the "use fucking everything on your character sheet to eke out an advantage" philosophy.
MP is used to prepare spells, then the actual casting of the spell is "free". It's pretty much the same thing that I wrote down here. It's still Vancian magic (and is virtually indistinguishable for the first few levels), but it has the advantage of being (a) more flexible, and (b) way easier to teach to newbies, probably.
I also want to stop the whole quadratic wizard thing by tying spell effects to "what level you prepare this spell at" instead of "what level wizard are you". If you want to cast a bigger fireball, prepare it as a level 4 spell instead.
This is probably the part of my kludge that is least compatible with OSR retroclone D&D. I will meditate on it.
FP is Faith Points, and its what clerics use instead of MP. I intend to differentiate it in a few different ways, like (a) clerics get more FP than wizards get MP, (b) prayers are less reliable than spells, since they might be ignored, and (c) you cast from a list of known miracles, instead of having to prepare spells ahead of time.
Other weird shit that I might include: I have no problem with wizards wearing armor (since armor has a bunch of drawbacks anyway, and fuck your tropes, because players will probably dress their wizard in robes anyway), people in platemail should automatically sink if they fall in water, wizards should all be forced to choose a sub-school like Necromancer, Illusionist, Elementalist, etc. Similarly, clerics should have to choose a god. And those choices should matter, gul-blangit!
Necromancers should have to spend a skill slot if they want to learn spells from the school of illusion. Clerics of the water god should have a reduced chance of success if they pray for a flamestrike.
Anyway, aside from those small blasphemies, it's pretty much the same D&D you know and love.
Here's a sample of a finished character sheet, for a level 1 dwarven fighter.
I am now open for comments.