Want to design a tabletop RPG? These open-source game systems are flexible enough to fit many different concepts and ideas for aspiring game devs.
Tabletop RPGs like Dungeons & Dragons are cool, but building a new tabletop RPG is even cooler. Designing a brand-new roleplaying game can be intimidating business, particularly when it involves creating original mechanics from scratch. For this reason, many roleplaying game makers search for pre-existing, open-source RPG systems that they can "hack" to fit the needs of their own games. The RPG systems listed below are particularly easy to hack, thanks to their intuitive, flexible mechanics and accessibility under Creative Commons licenses.
At the turn of the current millennium, the D&D publisher Wizards Of The Coast released a tweaked version of the rules for Dungeons & Dragons 3rd Edition - a system reference document called the d20 System. With this setting-agnostic ruleset, independent game designers could freely develop their own content using the rules of D&D, as long as they included the official d20 System logo on their cover and abided by the standards of its Open Game License. With this d20 System in their pockets, developers in the early 2000s came up with new, exciting RPGs, such as Mutants And Masterminds and The Star Wars Roleplaying Game, each featuring mechanics centered around the roll of a twenty-sided die.
The success of the d20 System kickstarted the development of many other "hackable" tabletop game systems, their mechanics carefully crafted by designers to be intuitive, easily tweaked, and easy to describe in a system reference document. In general, the more narrative-focused an RPG is, the easier it is for third-party developers to "hack," and the following game are no exception to this rule.
The Powered By The Apocalypse system was created by game designer Vincent Baker for Apocalypse World, a grungy, hard-scrabble RPG of bikers, warlords, and psychics trying to survive a post-apocalyptic landscape. It pioneered a new style of RPG gameplay based around straightforward character sheet "Playbooks" and lists of "Moves," specialized gameplay rules for narrative scenarios such as combat, manipulation, driving, and avoiding danger. When a Move is triggered by an event in the game's story, a player rolls two six-sided dice to see if their character succeeds in their endeavor, succeeds with complications, or fails in a way that still moves the story forward. By creating new Playbooks and Moves to fit a specific genre of fiction, third-party developers have made a plethora of new Powered By The Apocalypse RPGs, such as Monsterhearts, Monster Of The Week, Dungeon World, The Warren, and Night Witches.
The FATE system, developed by Evil Hat Studios, is itself a hack of an earlier open-source system called FUDGE, in which skill checks were resolved by rolling four six-sided dice with "+" symbols, "-" symbols, and blank spots on their faces. Designers at Evil Hat shook up the FUDGE system by adding two innovative mechanics to FATE: "Stunts," special abilities that expand the ways in which character Skills can be used, and "Aspects," descriptions of a character's virtues, flaws, and backgrounds that can be invoked by players to improve their rolls (or invoked by the Game Master to hinder said rolls). After Evil Hat premiered the new rules in its 1930s pulp adventure RPG Spirit Of The Century, the FATE system and its simplified FATE: Accelerated spin-off have been used for many other narrative games focused around high-octane action and clever gambits, some key examples being The Dresden Files Roleplaying Game, Ehdrigohr: The Roleplaying Game, and Diaspora.
Hardcore fans of the Critical Role tabletop livestream are likely aware of its new sponsor Cortex Prime, a tabletop RPG made by the designers of the D&D Beyond website. The system of Cortex Prime was designed to be modular and compatible with different genres of storytelling: By using special rules in the core book called "Mods," game designers and players at the table can throw together a novel set of character sheets, conflict resolution mechanics, and special challenges to fit the narrative they want to experience. The base gameplay mechanics of Cortex Prime are built around rolling sets of polyhedral dice ranging in size from four-sided to 12-sided. Players can generally add more dice to a roll by spending "Plot Points," a narrative resource similar in function to "Fate Points" from the FATE system.
Tunnel Goons, an "analog adventure game for nice people," is a small RPG created by the Highland Paranormal Society studio and released under the Creative Commons 4.0 International License for folks who want to share, adapt, or rebuild its mechanics for their own roleplaying games. The chief virtue of Tunnel Goons as a hackable RPG lies in the sheer simplicity of its basic mechanics: Players roll 2d6 and add the right ability score to see if they succeed or fail at a challenging action. Useful items add +1 to a roll. In combat, the damage a player inflicts/suffers is equal to the difference between their ability roll and the difficulty score they overcame. The genre of Tunnel Goons is modeled after the dungeon-crawling antics of old-school Dungeons & Dragons, but its many spin-offs (mostly published on the indie game site itch.io) cover genres ranging from "Demon Hunters In 19th Century Japan" to "Mycologists in pursuit of exotic, magical mushrooms."
