I would leave them free to run as light as they like in qualifying, but not allow any changes other than fuelling up between qualifying and the race.

To eliminate crafty tricks like using a qualifying set up in the first qualifying to set afast tome and then switching to a slower race set up, I suppose this might mean a single qualifying session. Alternatively they could carry on with the current "fastest n go forward" rules and put the eliminated cars straight into the lock-up. Teams who used a qualifying set up would run the risk that if they fouled up they would have to race with it.