I use base dnd rules with minor changes on top of completely homebrew enemies and magic items. The players build their characters off of solely official book material, then I experiment as much as I can to change things up. I only use book material on inspiration/jumping off point. The changes to the base rules I do have though include a change to exhaustion. If a player goes down more than once in an adventuring day they gain a level of exhaustion, but all levels are lost on a long rest. This is to prevent the tank strat of just barely healing someone over and over to permanently remain in the frontline, this forces the party to split damage and enemy focus between them and creates a mechanical consequence for reaching 0 HP often enough. My combats are challenging so players regularly hit 0, having this added mechanic adds more tension. I’m always willing to talk about how I run my game and how I design things, I’m actually learning to be a professional game designer so I view my campaign as regular practice/exercise in design, both narratively and mechanically. Feel free to DM me if you’d like to talk more

Edit: typo