GT used to make thermoplastic frames, threading a nylon/carbon tube through a set of Al lugs. It looked quite neat.
PBO is the latest wonder fibre, stronger than carbo or kevlar. It's used in some spokes.
A lot of bike design is limited by tradition, the UCI and component availability. Why are axles supprted on both sides? Why are frames symetrical when the transmission is only on one side? Why do we use large hollow axle BB spindles with tiny bearings squished into a BBshell designed over 100 years ago.