There's a whole different feel when the Prime Material Plane is the hub of the Outer Planes. It makes the cosmic neutrality of old school Druids and major Greyhawk character more understandable.
I go by the idea that characters with class levels are extremely rare, and have some kind of destiny. They're certainly not common enough to constitute a societal profession. Adventurers have, or will have, a heroic or dark legend tied to them.
If I ran a FR game where the planes came into play, I'd probably use the World Tree. Also, for the record, I prefer the pre-Deities & Demigods version of of the Great Wheel for Greyhawk. :p
This is what I've been contemplating in my nascent heartbreaker. Roll the ability score math into proficiency bonus, and have skills be background based.
An interesting thing about this thread is that it made me appreciate how good the changes to the Rock Gnome for the 2024 edition are. Now they are the iconic domestic fey; able to magically repair, clean and tidy up around the house.