What are different methods for map design?

I've been using unity for a while now
As i continue, I have found myself in a bit of a conundrum: making a map.

Now, I have already learned about the terrain editor that unity provides and I was actually using Maya to create realistic mountains using height maps (I like the Maya tools a little more than unity's). And then I transfer them into unity and using Object2Terrain. It works and it does exactly what I want it to do, but I feel as if the terrain editor has a lot of limitations to it and im missing a lot of different ways I can improve in map design.

What are different methods for map design? The map im trying to create isn't extremely big, but it is large (about the size of 4 football fields). It's suppose to replicate the environment in the mountains.

 

The Asset Store exists to provide tools that Unity doesn't provide by default. Take a look at Gaia and World Creator.