I recon its all down to the rear diffuser and the rear wing, with a little few tweaks to the front wing that are all thats needed to the cars, looking at some more responces.
Diffiser: The reduction of the 175mm line to arround the 150mm mark would probably be best for the diffuser, using current regs for the rest of the diffuser, it could work.
Rear wing: The reduction of the radii of the wing could be the best idea. Make it a standardised radaii, reducing the ammount of "scoop" that the rear wings have to reduce downforce. But id increase the size of it, a small ammount, maybes arround 50mm each side. Also reducing the ammount of height from the top of the wing to the bottom plane by 25mm would also reduce downforce.
Front wing: Increase downforce, but not by much, maybes by about 4-7%, try and get the cars to be "braver" into a corner, but id like to make the noses sort of standardised, but do away with the high noses we currently have. Something like the BGP001 or MP4/24 nose at the lowest or the F60 or FW31 at the other extreme. Try and get the cars to "punch" thru the hole in the air a little better.
Im shure there can be something done with the Brake systems that they can be standardised, maybes make it a mandatory rule that changing the brake ballance by the driver is banned, and can only be done in the pits by removing the front wing and changing it that way. But make it a mandatory rule that 60% of the bias must be toward the front brakes, whitch would be made arround 25% larger than the rear brakes. Theese with increased use of the dustbins, but id make it that theese can only be changed from launch spec once each season, as they are a expencive piece of equiptment with simmilar development costs.
Those are the 4 areas id look into more and more.