Design mode needs some more functionalty, e.g. merging 2 designs together:

Even in Sandbox the desig mode creates one extension after another.

So if I start design today, leave the design mode I have to save my Design.

But there are so many pages and fields in Business Central that makes it almost impossible to design all in one step.

So if I continue to design my environment tomorrow or later on it always creates a new extension.

It should be possible to choose an existing design extension when saving the new design. And then the older and the new changes are automatically merged together into 1 extension.

Also it shoud be possible to download the extension as an complett .app file to make it possible for non-developers to publish the design to the production environment. (Another option should be to activate designmode in production environment).