I can agree to most of what you said, particularly the part about well playtested core rules.
Here's how I think design should be partitioned and how much "Rule 0ing" should occur at each level.
Core:
-Allow for proper physical and mental description of a character.
-Give some basic and advanced combat options in a straightforward manner.
-Clearly classify all types of bonuses, effects and how they interact.
-Handle skills that would be expected in virtually any campaign setting involving humanoids.
-Allow for level advancement.
-Level advancement should not cause the Core game math to collapse. Ideally this would hold true no matter how high the levels go.
-The number of dice rolled (if any) should be low. (Personal preference, but it's a time thing).
-A system for determining how powerful antagonists should be.
Rule 0 Status: The GM can't touch the Core without explicit player consent. It should work very well, and players and GMs alike should trust it fully.
The Mantle:
-is applied to the core rules and modifies them slightly to suit a certain era, period, or style of game. For example, different Mantles for d20 include: D&D, d20 Modern, d20 Star Wars, d20 CoC.
A well designed mantle can be held to lower standards overall, since it has a higher degree of complexity but should still abide by the following:
-Set absolute minimums and maximums for spell/feat/technique/ability power and follow them closely.
-Keep power discrepancy between builds to a minimum (although some is probably unavoidable and maybe even desirable).
-Minimize power loops, recursion, and general cheesiness.
Rule 0 Status: GMs can be given some amount of authoritarian decree over what will and will not fly in the rules given by the Mantle. Unless something is clearly broken though, there should be player input.
The Crust:
- Campaign setting modification for world, time period or specific geographic location. (Eberron, FR, or Oriental Adventures for D&D; Past, Future, or Cyberscape for d20 Modern).
- Random supplements from web sources, and modules.\
- Splat books.
- Design should be focused on FLAVOR, although some crunch can be included.
Rule 0 Status: GM has complete authoritarian rule about anything in this regard, specially during in-game time. There is simply too much material here for every interaction to be considered, so brokenness will probably occur.
Ideally, this is how things would work. Ideally.
EDIT: Oh Great Fence Builder, I think I've derailed this one all the way to Hades. It would be much appreciated if the Rule 0 discussion could be set apart in a different thread.