big NA engines are a dying breed, the bench mark at the moment is the BMW mini cooper engine 1.6 turbo; already being used by peugeot in the RCZ and other manufacturers are either looking at it or making their own versions.
1.6 seems to be the magic number.
other things to look out for are dual FI setups that use both a supercharger and a turbo. means you can run bigger turbos with no lag as the supercharger works at the start then swaps to the turbo once the lag is ended. there is huge potential for tuning these kinds of engines. and getting more power is alot cheaper than trying to tune something like the V6.
this is just the start, in the future we could see things like gas turbine engines,