When creating a brand new TTRPG, the best place to begin is by defining your “High Concept”. This is the quick pitch that you could tell anyone and they’ll know whether or not the theme of your game are for them. For example, the concept for Cyberpunk Red could be “Cybernetically enhanced people fight to survive in a world ruled by corporations.” or Modiphius’ Dune TTRPG could be “Stories of political intrigue and ecological survival in the world of Frank Herbert’s Dune series.”

So lets begin with the High Concept for the new TTRPG I am making, Tamers of the Emerald Ocean. This game is built upon three overarching ideas; setting, characters, and gameplay.

The Character: In the most popular TTRPGs, character progression is dictated through a combination of gaining character experience (I get new abilities and can fight stronger foes) and item acquisition (I get a better weapon which makes me more efficient at fighting). In this model character experience is the primary progression track and item acquisition is the secondary track. Together, these two progression tracks allow players to create the play experience they enjoy the most.
Instead, I want to flip this and make item acquisition become the primary driving force for how players customise their gameplay experience. To facilitate this I began with the idea of having players use customizable mecha, but moved away from this since I don’t think I could do that concept better than other games that already exist in the genre. Then the final idea struck home. Pokemon.
By using collectable creatures, each with unique gimmicks and customisable power sets, I can create a progression system that focuses on items (the creatures) and still allows for the player character themselves to still be deeply customisable.
Concept: “By collecting and training monsters …”

The Gameplay: I have always been a simple man. My enjoyment of most TTRPGs come from the blend of exploring a world full of interesting stories and deeply interesting combat. Thus, any game I make will be exploration and combat focused.
Concept: “… to fight through and explore a world.”

The Setting: Inspired by the post-apocalyptic worlds of Mad Max: Road Warrior (1981) and Waterworld (1995), Tamers of the Emerald Ocean moves away from the blasted landscapes and drowned world of these setting towards something greener.
Massive trees grow from the earth so thick that they have created great canopies which cover all by the highest points. This swaying sea of leaves and branches have been named the Emerald Ocean.
Concept: “… a world flooded with oceans of plants.”

Thus we are left with a final, high concept:

“By collecting and training monster, you fight through and explore a world that has been flooded by an ocean of plants.”

And thus, we have the core themes of our game which all other elements will bend towards.