There are two changes the FIA could make that would increase overtaking 10 fold...
1. Ban carbon brakes
2. Bring back V12 or V10 engines
The advancement of carbon brakes is probably a big reason for the modern era of F1 having so little overtaking. The problem is they stop the cars too quickly, the braking zones are tiny and the majority of overtaking happens in the braking zone. If the FIA are going to start regulating anything then the brakes would be the best option, what team really cares about their braking systems being capped and regulated? They should restrict the use of exotic materials, and try to double the length of the braking zones.
As for bringing back the larger capacity engines, we only need to look at what happened in MotoGP to see why it's a good idea. When MotoGP switched to 800cc engines the cornering speeds increased. The riders had to maintain that speed through a corner because they couldn't just park it on the apex and power out of the turn anymore. That meant there was only ever one fast line, and if they attempted to overtake and failed, they would drop a long way back. MotoGP have realised that they need to restrict cornering speed, not top speed, which is why they are going to switch to 1000cc engines in 2011. The same applies to F1, the cars didn't get any slower when they switched to V8's, the top speed was reduced, but the cornering speed increased, and the more cornering speed the cars have the less overtaking there will be.