I care less about overtaking, and care more about overtaking attempts.
I think solving the off-line marble problem, as well as getting some support race cars that by their type, need to take a different line around the course (than the super tight F1 cars can) would help more than any aero scheme that the engineers will get around anyways... If you had a 1500Kg, closed wheel support series that ran 100 laps before an F1 race, you would have a very wide rubbered in line that would help the off-line passing attempts in a race.
That is what I would do to increase the overtaking possibilities in F1.