The original post is a pretty good list.
The only thing that caused me concern is:
* balance vs. niche protection: Each class will be specialized and fit a role at core.
Here, ‘each class will fit a role at core’ is the opposite of customizability according to character concept. So, it requires clarification about how to customize a character whose concept is beyond Gygax.
On a different concern, I was happy to see the need to support DM world-building.
DM world-building works best when the core rules are setting neutral − without any assumptions about what is in the setting. The setting might not even have other planes. So even references to ‘Fey’ creatures do well to avoid spacial assumptions. Especially religions, and different kinds of way of being religious, need to be more neutral and better model the reallife diversity of human religions. Let the DM decide without needing to erase texts.
At the same time, a separate box for official settings is excellent. These can show how each setting uses the core rules within its own unique context and flavor.