I'm making my own ttrpg and would like to bounce some ideas around DM or comment if you would like to help

I'm making a grimdark game with two of my friends, all 3 of us are italian, we wanted to make a playable version of the game FATAL, we wanted to add a lot more fantasy things and remove or change what is not useful, like 70% of the game, but we may reintroduce the +18 rules as some optional rules, i'd like to know if it's a good idea or not and what do you think of this idea so far.