Whatever you do it will be hard for F! to have overtaking under present layouts.
What I would suggest as a simple way of stopping the faster cars being held back is
To have one corner per track with extra width and seperated by a central white line, each driver would be required to take the corner on either the outside or inside of the corner and not cross the central line. each driver must use each side at least twice every five laps (except under the safety car)
Fail the two times and you must let the person behind overtake on the same lap or have a drive through
With one side being able to be taken faster it would open up more overtaking chances, but not make it certain to succeed
Minimal cost, fair to all
Simples