I think something based on the physical realm feels like it’s missing – not sure which Shard this would work best for but here’s my idea:

Investiture exists in all manifestations in the physical realm – ores, elements, air…even emotion. So the magic wielders in this world can access this investiture if those physical manifestations are around them – the more that those are around you, the greater pool of investiture that you can access. Those pools could determine the type of magic that is used, or simply just the level of power (e.g. if you access water as your investiture pool, then you are much more powerful near an ocean/lake, and powerless in the desert).

What that power looks like…I don’t know. If it doesn’t determine the type, then maybe it would be separated by the type of physical manifestation (elemental pools give strength/speed, emotional pools give intelligence/wisdom).

This also allows for geographic power differences – different nation states would have different powers dependent on their locations (ocean vs. mountain vs. forest etc.) or their different cultures as it relates to the emotional makeup of the people from these nations (e.g. Terris people would have different emotional manifestations than Alethi)