A few thoughts:

1. Introduce common body, thereby eliminating 99% of aero testing ($$$$) currently being spent. Do we really care about aerodynamics? No ROI for viewers. NASCAR has had success with this rule. 

2. Prefer 6 cylinder turbos instead of 4cylinders. BUT would like a more open engine development rules.  Isn't this more appealing to current engine manufacturers - Ferrari, MB, Renault etc?

3. Re-introduce fueling during pit stops & reduce tank size.  Balance of HP vs fuel mileage would make things interesting.

4. Like current tires with shorter lifespan, which has increased passing. Would eliminate mandatory use of both tire compounds. It drives me crazy. Only series with this rule......

5. Like KERS, DRS. Has introduced more passing opportunities.

Cheers