You need to be able to buy another teams previous year cars like they used to. Ok, the likes of RB aren't going to leave their very special bits on, but the cost of chassis development etc would be much lower for the poorer teams if they could just buy a previous year chassis and develop that themselves. Arrows used to just repaint the previous cars and run them as was, ok they weren't very competitive, but they still finished a few races.
Bring back re-fuelling too, it adds so much more to the races in terms of tactics, and it adds to the spectacle when the likes of Maldonado, Grojsean, Koboyashi etc run off with the fuel hose still attached