How I think game design should work is that you first conceptualize your game, second make the framework, third add the content. Meaning create an outline or document detailing every physical feature inside the game. And after the concept is written I think it should be a matter of following some scientific formula to fabricate the framework. And after the framework is made I think you should be able to use template based hierarchies to make limitless amounts of content.
I think it should all be a matter of putting the correct scripts/actors together in well constructed hierarchies... then copy/pasting those hierarchies and modifying them and importing your assets.