I am a product designer currently working on designing a plastic version of the British Standard National Beehive.
If you can come up with a cheap plastic hive, then I think you'll have a winner, if you can overcome the one biggest problem of plastic hives: plastic hives are often not strong enough for large hives, so the plastic bends and buckles, which means that the internal dimensions of the hives are no longer precisely correct... and with beekeeping, the internal dimensions must stay constant.
What would also be cool is if your design could be modular, in the sense that the hives are sold flat pack and are easily assembled. However, this would require even stronger plastic, to ensure that the hive walls don't buckle. I suspect, in fact, that a good plastic hive would be as bulky as a poly hive.
It would also be good if your hive is compatible with existing, wooden hives. On the other hand, having slight telescoping walls would make your hive more suited for travelling with the hives. Alternatively, don't make the walls telescoping but design a kind of strip that the beekeeper can attach to the place where the one box sits on top of the other box, which prevents the boxes from sliding off each other. Then your hive will be good for travelling and will have beespace that is compatible with wooden hives.
If you use the pre-war design (that is still used in the Netherlands with their spaarkast hive), and make the hive 465x465 mm on the outside, then you can make the hive easily out of the four identical walls plus the two frame rests that fit into small slots inside the four walls. If you do that, then you can also sell smaller frame rests that can be used with the exact same walls but which will allow the beekeeper to use Commercial frames, and bigger frame rests that will convert it into a Smith hive.
It would have been cool if you could somehow allow both Commercial and National box heights with the same generic components, although I suspect that you would not be able to do that.
I know nothing about plastics, but would it be an idea to make a hive that has thick, strong, sturdy corners, so that the four walls don't actually carry any weight, and the corners carry all the weight?
Plastic has many advantages over e.g. poly. Plastic does not let any water through nor does it absorb water. Depending on the type of plastic, you can wash it and clean it of all disease easily in the dishwasher. You can make the plastic walls "hollow" and fill them with an insulating substance. I suggest you take a look at the Turkish plastic hive's marketing materials.
