Go about designing a world map for game engines I have been modelling with blender for a year or so and I am starting to work on a pretty large project, something of such scope I have never attempted.  

I have created a really rough mock up for a world map I want to create. I am having a hard time figuring out how I should tackle the final product. It is about 2km square with a slight incline and on one side is ocean. I want to eventually import it into unreal or unity. 

Should I create a rough layout of the entire terrain and place the roads, buildings and trees on top and add details to the terrain where I need them?  Another way I was thinking was to add the buildings, place the roads, parking lots etc in between and only add terrain where it is exposed? 

Or is there a better way?
There is indeed a difference between having one giant soil and placing elements on it and dispatching pieces of landscape where it is needed. Since you have an incline, I suggest the first. I strongly reccomend that you work with armory3d which is the pbr engine for blender. Thus with arrays and displace you can have a soil, add another displace with a texture for the paths/roads, use the nodes with normal -> dot connected to mix shaders via colorramps to get different surfaces types depending on the slope. Even add some tesselation displacement and you'll get quite near to photorealism. The next step is to create/get a grass shader and some vegetation models and dispatch them on that land with a script or grease pencil. That's for the natural part of it.