How does Lego come up with set ideas?

I don't really know. But I think I read somewhere that they're open to suggestions. The designs are built to be practical and fir the shape and other details to be realistic. But then, if not practical enough, changes must be made. Also, designs can't be to complex large or expensive because to complicated might annoy some users but more older users sometimes enjoy it. Obviously the less expensive the more they sell but the more complex sets use more pieces and therefore are all larger. Hence mor expensive. I think I answered your question but I do get side-tracked and carried away very easily. Just trying to help.

:)