A quick glance a the bottom of this post completely answers the question about "outside" ideas . . . Terraria may have more fan ideas than any other game I know.
That said, there is a difference between adding a concept or idea, and then adding a completely designed and fleshed out biome created in someone else's vision (in this case, zoomo). It may or may not match how the Relogic team would want something like this to be. Not adding the Cyber isn't a limitation of manpower, but a clash between creative originality and just "filling your game to the brim with other people's vision".
