There is no perfect game system that captures everything we want as GMs. As such, I take great satisfaction and enjoyment out of tinkering with rules modifications. However, one thing I have learned is that it is vitally important to have most (and ALL of the major) rules changes defined up front for players.
Many players like to tinker as well, playing within the rules to find and create the optimum character that fits their vision. What may seem like a minor change in the rules can dramatically change how a character plays, and cause great distress to a player.
That being said, here are house rules that I utilize:
- D&D 5e Adventurers in Middle Earth
- Star Wars RPG Saga Edition
- Star Frontiers