Fortunately for them they can do few creative things to make the current areas more lively with some effort.

For example the increased beastmen activity in these areas could help in making them more diverse. They can also add giant NM's to roam around and work as "landmarks" as themselves. Later on they can do the same with Garleans and their machinery.

I think it'll take quite a bit of effort to get these areas filled with interesting details, but there's no need for tons of costly assets (although they'd definitely help too).

At this point I think they should focus on a few large unique landmark assets in every open "zone" that you could see in the distance. Then just fill the rest of the place with beastmen, garleans and NM's.