The key factor is what power output do they want. If RS5 stays under 500 hp SC or turbos are wastage. Otherwise they are necessary. My choice would be 4.0 TFSI at around 1 bar of boost and ~500hp. But if Audi follows the way they did with RS6, it can even be turbocharged under 500hp to let room for tuners. Maybe this is the most likely option. Personally i don't think if we will see many N/A RS cars in the future but maybe i'm wrong. I wonder why Audi made the S5 evo then.