[DAT]

To think a little outside the box ...

Why only one?

You can have each city state master a different magic:
e.g.,
City State X - RPM
City State Y - Path/Book
City State Z - Sorcery
etc.

Or different racial groups:
e.g.,
Humans - Basic
"Elves" - RPM
"Dwarves" - Path/Book
"Trolls" - Sorcery
etc.

Or, if you really like RPM, but haven't decide on the specifics, let the specifics vary by City State or Race:
e.g.,
City State X / Humans - Basic RPM
City State Y / "Elves" - RPM with High Thresholds
City State Z / "Dwarves" - RPM with Charms
etc.

If you setting does not have "Elves" and "Dwarves", replace the races with different ethnic humans or your alternative races (Fishmen, Insectmen, Reptilemen, etc.).

But one other important thought to bring up - what are your players up for?
If you create 40 pages of magic rules for your uber RPM, do you have players who will read it?
As a cautionary tale, a friend GMed a group using complex RPM like magic system (not GURPS). Among 7 or 8 players, no one really want to be a mage and have to learn extensive rules. So the party had 2 "rogues" and 5 or six "fighters" ...
-Dan