I believe the wheels are CNC machined, not cast. Roin already manufactures the 17 inch wheels; thus they already have the CNC program (or molds) for the 17 inch wheel. They only need to redesign for the 18 inch wheel. And if the wheels are manufactured using a CNC machine, once the design for the 18 inch wheel is settled upon they should be able to produce both the 17 inch wheel and the 18 inch wheel in any width or offset, for front or back. Thus 17/17, 17/18, or 18/18 sets should be possible. For the best looking 17 inch/18 inch set the 18 inch wheel should match the 17 in version as closely as possible, i.e. the 18 inch wheel should be arrived at using the same strategy that was employed for the 17 inch wheel.
The rear wheel can have short ribs, long ribs, or no ribs at all. I am completely flexible in that regard.
Once we get beyond that stage, the wheels a particular person orders should be defined based upon the tires which they intend to use. Since wheel diameter is limited to 18 inches my personal preference would be the wheel and tire set below:
Front Tire: 245/40ZR17
Front Wheel: 17 diameter, 8.5 width, 16mm offset
Rear Tire: 275/40ZR18
Rear Wheel: 18 diameter, 9.5 width, 8mm offset
There are 70 tires available in the front size on Tire Rack, and 40 tires available in the rear size. There are 18 tire “SETS” available in these sizes on Tire Rack. The set should remain available well into the future. The links below list all the cars that use tires in those sizes.
If I had to choose an 18 inch front wheel I would opt for:
Front Tire: 245/35ZR18
Front Wheel: 18 diameter, 8.5 width, 16mm offset
and use the same rear tire & rear wheel. There are 12 tire "SETS" available in these sizes on Tire Rack.