Do you design a rollicking good adventure? Stock it was amazing monsters, traps, treasure; some drawn from mythology and literature while others are made up from your personal nightmares? Do you cram as much fun and imagination as you can dream up ... then figure out how to make it all run using whatever rules you have handy? D&D, T&T RQ, EPT; no matter. It's the adventure, the rules are just the means to that adventure.
Congratulations! You're playing it old school.
If you design your adventure around the rules, its monsters, concepts, tropes? You should keep trying. It isn't the rules that make the game, its the campaign that brings the rules to life.
It's the adventure, not the rules.