Save them out a prefabs. You should build complicated structures only once and then save and reload them as needed. Add a player start entity. Light as you go. Don't build all the architecture in the map and add light as an afterthought.
Dramatic lighting is key to visually exciting maps.
Learn how to get you want out GenSurf. You can manipulate brush vertexes later, but getting it mostly the way you want it in GenSurf will save you a lot of work. Try to develop your own terrain texture combinations. Remember that even though maps can be larger, care must be taken to control how much can be seen at one time. Long, distant views and complicated architectural structures do not mix well.