Alternatives to stacked iron plates for electric motor cores?
The lightest stator core material is nothing - literally.

In a coreless ironless motor the stator coils are wound around a non-metallic frame, glued to a surface, or even suspended in mid-air with the coils themselves forming the motor frame.

The motor below weighs 28g and produces 83W at 60000rpm (100W input power, 83% efficiency). The frame is made of plastic, and the rotor is a cylindrical Neodymium magnet. The only 'iron' in this motor is the shaft. 



Here's a tiny home-made motor which has a 0.5mm diameter rotor. The shaft and bearings are held in place by the windings. The black rectangle glued on top is an A1442 BLDC controller IC with built-in Hall sensor.