I'd like it to have a simple, minimalistic interior, 3 litre straight 6 engine, and also to come in coupe body style, two doors, no B pillar coupe. What I would really love to see, and I don't think anyone mentioned is a possibility to choose between things when ordering a new car, such as old school key ignition vs start button, manual seats vs electric in all trim levels, metal vs glass sunroof, steering wheel without buttons vs one with, regular cable handbrake vs electronic one and such things. Sort of a purist version of the car.
I wouldn't like to see downsized engine and too many screens, ambient lights and such. There is absolutely nothing wrong with analogue instruments and physical switches.
Now, when I read what I've written it's obvious that I want a new RX-7 FD with a straight 6 engine.
A new Mazda 6 that is a bit like MPS (Speedsix) and a bit like RX7 is actually a really good idea.