It's annoying that the Open Gaming License 1.0a is under attack, but it's not actually detrimental. As many people (and in fact possibly most people) recognize, you don't need anybody's permission to play a game at home, nor to write an adventure that happens to work with D&D™ 5th Edition. Don't copy 5e rules verbatim, and you can even write rules that just so happen to work with 5th Edition, and you don't need a license or permission.
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For instance, say you want to invent a minigame for 5th Edition. You want a quick way to let your players gamble, so you flavour the game as a card game, like Blackjack. Call it Knucklejack. The first draft of the rules go something like this:
Not bad, but there's a lot of 5e lingo in there. Whether or not phrases like "Dungeon Master" and "Sleight of Hand" and "Proficiency Bonus" intrude upon any company's trademarks is debatable, but they definitely lend the appearance that your minigame only works in a specific system. By tying your minigame to D&D™ 5th Edition, you're actually limiting your potential audience. Sure, most Game Masters can translate the code from one system to another, but why introduce that dependency when it's not required? Here's a second draft that's essentially universal:
That's pretty generic. Fully compatible with D&D™ 5th Edition, but also with Pathfinder 2, Project Black Flag, 13th Age, Swords & Wizardry, and a lot more. Heck, you could even go a step further and remove the specifics about the kinds of die you want players to roll. The players don't have to roll a d20, and the goal doesn't have to be 21.
It's a fully universal rules supplement with no dependency on anyone's license.
Suppose you want to write a rule that interacts with a specific subsystem in an existing game. For instance, maybe you're designing a spell that specifically reduces a character's Passive Perception. Lots of games have the concept of a "spot check" or some mechanic that provides the opportunity for a character to notice something without necessarily knowing to look for it. But Passive Perception is a little unique. The Game Master is supposed to just know everyone's Passive Perception, and make judgments based on that number.
Is it possible to create a spell that affects Passive Perception without being obtuse about it? In other words, you don't want to write something like this:
Reduce target's inactive noticing value by 1d6...
Luckily, you don't have to. Sure, mechanics get names, but they're ultimately a form of theoretical machinery. In the case of Passive Perception, the components are 8 plus Proficiency Bonus plus Wisdom. You can target those components without treading on arguably trademarked terminology:
Reduce a target's Perception bonus by 1d6 when it's used for anything not requiring a dice roll.
The only kind of Perception [non] "check" that doesn't require a dice roll in D&D 5th Edition is Passive Perception.
(I'm not convinced that Passive Perception actually works all that well in 5th Edition, and I actually hope Project Black Flag fixes it, but I digress.)
If you're worried about using everyday words in an arrangement that mimics D&D 5th Edition too close for your comfort, that's fine. You can redefine terms. It might feel awkward at first, but if you do it consistently in everything you publish, then your reader eventually gets comfortable with it.
In a book of traps I published with a gaming buddy of mine, we wanted to be vague about damage because we didn't know what level characters would be when they encountered each trap, much less what system would be in use. So instead of assigning damage values, we assigned damage severity. At the beginning of the book, we specified that damage was up to the Game Master, and gave this example for 5th Edition:
We did the same thing for difficulty. We did use the term "DC" but we didn't have to. We could have used "difficulty" or "target" or "threshold". For one system, an "Easy" difficulty might be DC 5, while for another system it might be DC 10, or "3 hits", or whatever terminology the Game Master needs to slot in for that game system.
Game terms that are just normal everyday single words, like Strength and Dexterity and Constitution, and so on, can't be trademarked because no company owns those words. But maybe you want to set your game apart from the system it happens to be compatible with. It's just a matter of establishing vocabulary. For instance, suppose you had this at the start of your book:
If you're a dnd player, there's no doubt which character attribute maps back to the published stats in the D&D™ rulebook.
In the end, writing content for all systems often means you get a theoretically wider audience with the same amount of effort. It also means that your work can theoretically endure well past specific editions. Everything I've written here applies equally to Pathfinder 1st Edition as it does to Pathfinder 2nd Edition, and also for Starfinder, any number of OSR games. Most of what I've written here can be used with Shadowrun, Call of Cthulhu, and any other game that uses imaginary people and real dice.
You don't have to make your adventure or supplement "generic" for it to be universal. Establish degrees or severity early in your publication, and let the game master translate the specifics for whatever system's being used. That's what game masters do anyway. Game masters are constantly adapting things that don't work for their gaming group. An adventure says the Big Bad is in the room to the left, but the game master forgot to mention there was a door to the left, so now the Big Bad is in the room to the right. Wait, the controls for the planet-killing bomb was in the room on the right. What am I supposed to do with that now that the Big Bad's there? OK, the bomb's concealed in a demiplane inside a magical safe, under the rug. Right where the Big Bad is standing. Defeat the Big Bad, diffuse the bomb, save the world. What's the difficulty check to diffuse the bomb? DC 20. No wait, this is Starfinder. DC 30. But what if my reader's playing Shadowrun? Formidable.