After working with this stuff for a decade I observed that there are common elements in the RPGs that are consider compatible with one or more editions of DnD. I found it useful while working on my Majestic Fantasy Rules to keep these in mind as I develop various subsystem. There is no right or wrong way of doing this but it is helpful to have a starting point.
My view of what constitutes a minimum set of mechanic for a DnD related RPG are:
- Six attributes: Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma generated on a 3 to 18 scale with 10 being human normal average.
- Saving throws to avoid bad things.
- Armor Class as a target or an index to a chart to see if damage is scored.
- A d20 to-hit roll
- Difference races/cultures that offer a package of attributes bonuses and abilities.
- Experience is represented by higher levels.
- Classes that are a package of abilities arranged by levels.
- A character's health is represented by Hit Point when brought to zero incapacitates or kills the character.
- Creatures can have hit dice instead of levels.
- Creatures at a minimum have hit dice, hit points, movement, armor class, and a list of special abilities including attacks.
Beyond this anything is fair game. As long the above list is implemented it will be highly likely that the game will be seen as DnD compatible.
The interplay of the numbers used for the to-hit roll, armor class, hit points, and damage is a large part of what gives a specific edition their flavor.
You need to keep this in mind because the numbers work out differently for ODnD, ODnD+Greyhawk, ADnD, ADnD+Unearthed Arcana, Holmes Basic DnD, B/X DnD, BECMI DnD, ADnD 2e, ADnD 2e + Skill n Powers, DnD 3.0, DnD 3.5, Pathfinder, DnD 4e, and DnD 5e. The good news it is not rocket science. Just need to figure out what edition you want it to be like and go from there.
Simplifying things even further the above can be grouped into broad categories:
- Classic DnD (ODnD to ADnD 2e)
- DnD 3.X (DnD 3.0 to Pathfinder)
- DnD 4e
- DnD 5e
If you noticed I didn't mention anything about specific classes, spells, magic items, lists of monsters, etc. To me these are setting details, either specific settings like my Majestic Wilderlands, Tekumel, Blackmoor, or Forgotten Realms. Or the generic fantasy that the core books of most editions of DnD assume.
With stuff like Dark Sun, Spelljammer, Eberron, Lamentations of the Flame Princess, Arrows of Indra, Spears at Dawn, and other worthy works, I think it been established a RPG can be considered DnD even if it depicts a radically different setting or different vision of the fantasy genre.
For most of these games this was accomplished by having a different set of class, items, monsters, and even different systems of magic.
The point of this post is to offer a useful starting point from which to develop your own take on the world's most popular roleplaying game.