If anybody is thinking of designing some new concept 3d printer, I would like to ask you to consider if this is practical.

Have you considered implementing a "forklift" mechanism? I mean specifically the type used in tall warehouses, that can extend twice the forklift's height? This will allow you to extend the printing size of your 3d printer to 'twice' its size.

In the future, if we get really accurate and good at making 3d printers (with printed electronics, motors etc...), you can perhaps even print the same printer within it in one go.

This video is a forklift lifting hay very high: [www.youtube.com] Just down size the mechanism to something that will fit a desktop 3d printer.

For practicality purpose, your first 'forklift' based robot should extend only the z axis. But if you are brave enough, try extending both the xyz axis (Not sure how you could deal with the platform tho).

This is a forklift robot. Imagine it's 'fork' was a field 3D printer, printing concrete: [www.youtube.com]

I think it could be done, but I don't think the added complexity is justified by keeping the machine small until it's printing. There are some pretty straightforward folding designs for portability, if that's why you want to keep it compact.