Where do you tinker with game systems?
I don't want to know where your GM Tower of Solitude is or anything, I want to know where you think the fiddly bits are in your game system of choice. Does your tabletop system have a rigid design with flexibility to create what you need on the fly within the rules? Or does it have a flexible design that encourages you to personalize the rules before or during play?
Under this idea, the rules are designed to change to fit conditions during play. You can add new spells, new skills, or new classes easily, and in extreme cases you're encouraged to swap out entire systems or subsystems to fit your game or your GMing style. Supplements usually include setting, but they provide new optional rules to support something unique to each setting.
Systems with mainly flexible design include D&D (yes, even 4e, though it's more Rigid that other D&D versions), most OSR RetroClones, RoleMaster, Cyberpunk, Shadowrun, Traveller, and GURPS. What else comes to mind?
Under this idea, the rules do not change and everyone is expected to follow the rules to the letter. However, the system creates space so the endless flexibility in RPG play happens within the rules and not by changing the rules. House rules may exist, but they're few and far between since the base system provides all the flexibility you need. Supplements may exist, but they tend more toward setting elements (Fiasco Playsets, for instance) rather than additional rules. Many rigid design systems have no supplements or extremely limited supplements.
Many modern RPGs fall into this category, including Apocalypse World, Dungeon World, Fiasco, Leverage, Marvel Superheroic Roleplaying, Fate, and Burning Wheel. I know I'm missing systems here.
The Hero System fits in both camps, I think. The point buy power creation system is a great way to balance very different abilities and very different character styles and have it all work in a single system. The power creation is very Rigid - it contains the way to make anything within itself and it doesn't need much of anything else. However, supplements have come out covering new Skills, new items, new backgrounds, and new Talents. So yeah, the Flexible/Rigid paradigm breaks down in certain cases.
Why Should I Care?
Each design system carries different expectations, specifically around house rules and the role of the GM during play. In a Rigid system, the GM's actions are much more restricted, and in some cases they're even listed for the GM to easily select. In Flexible systems, the GM interprets the rules and can introduce new mechanical systems on the fly. People trying to talk about the GM's role in RPGs need to be aware of what design philosophy they're working from, because the ideas are wildly different. Even what specific words mean or which words are taboo (GM Fiat isn't necessary in a Rigid Design game, and in extreme cases using fiat could break the system) change depending on what design system you choose to represent.
So where and how do you tinker with your game systems?