For starters, I'm very similar to Uggor in that I prioritize the theme heavily. I saw an opportunity to do interesting things based on D&D, and considered that my first priority. To that end, everyone needed something interesting to do. This led to roles being classes. We ended up stumbling over the concept of core class identity about a quarter of the way through the role cards when I started writing Wizards and we realized spell memorization could totally be a core class mechanic. Danner came up with the Rogues' Caution mechanic, I countered with the Paladin's Fall/Temptation, and it just went from there. Warlocks have Boons, Bards have Inspiration, Barbarians have Rage, Clerics have Spontaneous Healing, Fighters take Leadership, etc.
But, in the end, I didn't want it to be a role madness game, as I ultimately wanted the game to be day-driven, not night-action driven. So I made each player's "something interesting" mean very little in the grand scheme of things. I needed mechanics that would reward clever plays (Arialla's framing) and accurate player reads (Xanjori's track that completely wrecked Arialla's framing), as anything less would render actions useless at best, or counterproductive at worst (such as if Senna had stolen Catta's Misty Escape).