I actually just finished interviewing Brendan Conway, the lead designer behind Masks: A New Generation and Avatar Legends, about this subject! He said that he personally likes to design a concept first, and then build mechanics around that concept - for example, in the Avatar shows characters always have a narrative arc full of internal struggles as they grow as people, so in Avatar Legends there is a Balance track that builds those internal struggles into a concrete game mechanic. I wholeheartedly agree with this train of thought - my process for Gardenborn has been to define what I wanted to write a game about (plant-based chivalry stories) and then built mechanics using those elements and different concepts I've heavily modified from other games (for example, my buddy Finch designed a randomized weather system for D&D, and I've modified my own hack of the system to instead note the level of plague infection in every part of the forest). I'm not done yet, but that's been my main line of thought going into this project.