They can keep the current engine regulations, just run unrestricted fuel flow and bring back refueling. This way, we still get the power we want, but the teams don't have to spend a ton to go over to another engine regulation.
Keep the current down force rules, just position the rear wing lower, and allow the manufacturers to do whatever kind of nose they want to as long as it is safe. The larger rear wheels is a great idea as well.
Not saying that this next thing has to be done, but if they really want to cut costs, mandate spec rear and front wing pieces. The teams can adjust them however they want, but everyone will have the same possibilities.
Take away DRS altogether and take away more down force. Make the drivers wrestle these beasts around the track. Also take away all fuel mixture settings so if a driver needs to conserve fuel, they have to short shift or manage throttle input on their own.
And a note about balancing performance and cutting costs. Mandate that at the end of every season, every team must share every piece of information from the past season. And to make sure no one is hiding anything, make the teams submit every ounce of data to the FIA so the FIA can guarantee everything is being shared. The access to this means gaps will close and teams may decide to spend less on developing their car considering everyone will have access to it at the end of the year.
Just some of my thoughts, take 'em or leave 'em