I have a hand cart / furniture dolly that's almost entirely plastic. The only metal bits on it are the axle and the handle. It's perfectly durable and I've used it to cart around items that weigh 600 lbs. (300kg).

It's also a good little example of where plastic is good and where it's not. The axle and the handle get the largest amount of stress, so they remain steel, while the body of the cart can be a trussed plastic mold to achieve the strength needed.

Portable gas cans are another area where plastic wins in my book. It's lighter, more impact resistant, and cheaper to mass produce. And it don't rust! Sure, there are probably niche cases where you want a metal jerry can, but for most applications a blow-molded plastic container is better even if it wasn't also cheaper.

But I still think a cheap, strong, and corrosion-resistant (or -proof!) metal would be revolutionary. For all the structural uses where plastic won't do. Vehicle frames, airframes, bridges, tooling, etc. My holy grail here is something that's basically aluminum, but without the fatigue issues. Oh, and closer to steel in cost.