What rules should I take from each D&D edition?
Tweaks to your game that don't create extra work.
Some rules depend on others. Changing one could have unanticipated knock on effects. Other rules operate independently of the others in the system. Here are a list of rules that operate independently enough that they interchangeable, without unintended consequences. They impact the game by degree, not kind. They make the game more or less deadly or more or less random. They may expand player choices.
Any edition
Lower or increase a class’s hit die. In Basic/Expert, thieves and magic-users roll a d4 for hit die, clerics roll a d6, while fighters roll a d8. In most TSR/WOTC editions, these dice are increased by a step. Thieves or rogues roll a d6, clerics roll a d8, and fighters a d10. In some editions, wizards roll a d6 instead of a d4.
It’s rare for other rules to hinge on what hit die a character is using, so you can bring B/X in line with the other editions and make player characters a little tougher or you can make 3.5 or 5e more deadly by lowering hit dice to those found in B/X. Or you can go further in either direction. Wizards could be d2, rogues d3, clerics d4, and fighters d6 if you like. It’s a dial you can turn without worrying about unintended consequences.
Ability score generation methods. Any method from any edition can work in any other edition. Changes increase or decrease randomness and power.
Ability score modifiers and their rate of improvement. Since 3rd edition, ability modifiers increase/decrease for every 2 points in the ability. 10-11 is +0, 12-13 is +1, 14-15 is +2, etc. In earlier editions these weren’t as linear. You could use the Old-School Essentials system where a 9-12 is +0. A 13-15 is +1, 16-17 is +2, and 18 is +3. You may need to extend the system into the higher numbers if you this in a game that allows that. You could also go the other way and use 3rd edition’s modifiers (and 4th’s and 5th’s) in Basic D&D. Changing these determines how much ability scores matter, but the change is one of degree and won’t have major unintended side effects.
Alignment systems are interchangeable. You can change Basic D&D’s trio of Law, Chaos, and Neutrality to AD&D’s 2-axis system of nine alignments or vice versa, with minimal unintended consequences.
Old-School editions
Measure time spent exploring dungeons with 10 minute turns.
Reduce the number of hit points gained once you reach a certain level, such as 9th level in Basic/Expert D&D. After that point, don’t roll hit dice any more. Instead, add a fixed smaller number depending on your class. This keeps the hit points lower at high levels.
Drop the identify spell. Old-school editions didn’t have it. To find out what a magic item does, you have to glean clues from its description and then try it. Or you could hire a sage to research it.
B/X and Old-School Essentials
Slot-based encumbrance (from Carcass Crawler 2)
Detect evil determines evil intent, not the evil alignment, as it does in certain other editions.
Rolling to light a torch or candle during combat. You must roll a 1 or 2 on a d6 to light a torch in combat. This is your action that round. You can use this system or can change it to a skill check such as a Survival roll in an edition with skills.
AD&D 1e/OSRIC 3.0
Medium and Small characters take up 3 feet side-to-side. They can require 3 feet, or more if they are using a weapon that requires more space, like a two-handed sword or heavy warhammer. This rule allows groups to have 3 columns in a 10 foot wide corridor.
Death at negative 10 hit points.
3/3.5e
Ascending Armor Class, obviously. Descending AC isn’t hard, but ascending is easier to learn. And both are preferable to using matrices, which take up far more space in books, character sheets, and DM screens.
Maximum hit points at 1st level if you want characters to be a little more durable. It won’t change the dynamics of earlier editions too much. Early modules were written for more player characters than is common in a party today, so maximum hit points compensates if your group is smaller.
“Confirming” critical hits. If you want critical hits to be less frequent than in 5e, you can adopt the rule that critical hits must be confirmed by a “critical roll.” This is a second attack roll with the same modifiers and target AC as the attack that scored a crit. If this attack would be a normal or better hit, the original attack is confirmed as a critical hit and gains the bonuses of that. This can work as a middle ground between the early editions that didn’t have critical hits and the later editions where they were more common.
Weapon critical hit properties. These determine how much extra damage a critical hit with the weapon and what range of “natural” results on the attack roll score a critical hit or possible critical hit. For instance, a pick multiples damage by 4 on a natural 20, while a scimitar multiplies it by 2 on a natural 18, 19, or 20. The way the multiplier works can be adopted to be consistent with the rules of the edition you are playing. If playing 5e, then it could multiply the dice of the attack and not the modifiers.
5e 2014
Equipment packs. These speed up character creation a great deal. I would make them more modular or increase the number of choices. I would add a pack of writing supplies. Adapt the prices for the edition you are using. The discounted price isn’t important.
Average hit points. When leveling up, instead of adding a random number of hit points to your maximum, add the average of the roll. In 5e, the option is to round up, so a character with a d8 hit die, gains 5 HP. But you could change it to round down (4 HP). Rolling hit points isn’t the action in any edition the game and it doesn’t have to be random if you don’t want it to be.
Advantage and Disadvantage. These increase or decrease the probability distribution of a roll without increasing or decreasing the maximum and minimum rolls, unlike adding a bonus or subtracting penalty. You can add this into a game without it if you want that math. You can still use all the bonuses and penalties you want to represent other factors, such as those that shift the whole curve.
Ready action. This version of the readied action is succinct and flexible and can be added to other editions without unintended consequences. It’s utility can be reduced if you are rolling side initiative each round, since the side that readied an action might get another turn before their opponents, but it still works fine.
Death saves. If you like these (I don’t), you can add them to any edition without conflicts with other rules.
Success at a cost (DMG). If a player fails a roll by 1 or 2 on a d20, the DM can allow them to succeed but at a cost, such as dropping their weapon, falling prone, etc.
5e 2024
At character creation, a player can choose for their human, tiefling, or aasimar to be either Medium or Small size, rather than limiting them to Medium size. This option could be extended to any Medium size ancestry.
Choosing languages, rather than languages being determined by species or class. If a character is raised among a different species or culture, they may not know the language associated with their own species or culture. Language is cultural and culture is learned and shared. Druidic or thieves’ can can still be linked to class.
Adding Common Sign Language as a language option.
The 2024 Exhaustion rules could be added to most other iterations without out any trouble.
The Bastion rules. The system is largely independent of the rest of the game.
Retraining class features each level to choose different options (introduced in Tasha’s Cauldron of Everything). This can reflect practicing different abilities.
Dungeon Crawl Classics
The dice chain. You can bring it into any game with target numbers. It works best when you have more weird dice.
Pathfinder 2e
Beating a target number (such as AC) by 10 or more results is a critical success. Failing by 10 or more results in a critical failure. This allows critical hits and failures to depend more on skill and less on chance. A natural 1 and natural 20 are also a critical failure or a critical success, respectively.
Conclusion
What other rules can be interchanged without needing to rewrite abilities, features, spells, and equipment?
Imagine a game that mixed and matched all of these rules into some never-before-seen combination.