What I mean by plastic-like is biodegradable materials that replace a purposely designed subset of these functions of things plastics are really good at:

Waterproof

Variable stiffness - Flexible / Rigid

Airtight

Lightweight

Capable of being colored

Injection Mold process capable

Variable opacity - Transparent / Opaque

I think cellulose/lignin/chitin based materials could replace many of these functions, there's some progress on that front but not nearly enough to chase down plastics.