I'm making two of my own systems for two separate games at the moment, though I'm really focused on one alone.
Basically, I think that mechanics inform how people play a game . People respond to incentives, and they respond to incentives based on what options exist for their characters. I want to make sure those options tell thematic and interesting stories.
Mechanics form the "words" for the language of a game, and I haven't found any preexisting games that let me speak to players and encourage them to play out certain types of stories. Most languages in the world for games are focused pretty narrowly on combat, and I want to have a language that encompasses more than that but deals specifically with the themes and tone that I want the games I'm making to have. Adapting PbtA, or FATE, or d20 to them would leave the gameplay loops absent from the game without the GM trying to force them in, for relatively minimal payoff for the players.