The optimal gap is when a fast car is behind a slower car. But because most motorsport gifts the fastest driver with the pole, and the second slowest further back the field just spreads.
So, to generate passing you need either unreliability (cars starting near the back, or losing time in longer than intended pitstops), reverse grids or part grids, or regulations that mean that the cars' speeds vary a lot over a race.
Modern cars are reliable, so that's pretty much the first one out. Reverse grids aren't very fair unless you have sprint and feature races (that surely no F1 fan would want), and the cars speed is fairly consistent even when they made them use the same set of tyres, or forced to use 'option' tyres for a stint.
The cars are over regulated and over developed, so even when faced with the crap tyre compound they aren't usually much slower. And besides, they've all worked out to less than a second accuracy which 'strategy' is best, thus ensuring that in all but the rarest of cases the fastest cars finish in that order. Which doesn't promote overtaking.
Slicks (which the teams need to get used to), a testing ban, hopefully a larger difference between compounds, and gimmicks like KERS might mean that the order is shaken up a bit more. But assuming that won't happen (and it'll be rare) the increased field spread will be the dominating factor.
Personally I think that qualifying needs to have some points and be on low fuel, and then a random number generated between 1 and x (where x is the number of cars on the grid) defines where to reverse the grid. This number is drawn 10 minutes before the race, so the teams don't know where they are on the grid, and hence can't optimise everything as much. The fastest qualifier might be on pole, but also might be last. How much fuel should he put in the car, and should he put on more downforce for laptimes or less for straight line passing ability?..