What you think?
They look nice, but they look like basic world machine presets (its trivially easy to get splatmaps out in 2.3), and not great for gameplay. Looks like I could pretty easily see/walk-off the edge of the map. Lets see a world with thoughtful boundaries, and populated with foliage, roads, paths, and settlements 
They all look great! Just add props, vegetation n roads then they’ll look more lively and game ready.
how would i had boundaries, so you don’t fall off. would you just arrange rocks with different angles and size along the sides.
That last one looks like it could be the begining of something - toss in a cave system, some multi-level buildings, etc… you might just have a cool level!
Sure, natural boundaries like rocks, dense foilage, maybe a foggy area with colliders, swampland or water bordering a side or two, mountains … stuff!
You could always just make an island
But otherwise, I would use layout generators in WM to make impassible mountains around the edges of the terrain; or you could sculpt a “base” terrain in an app like mudbox, zbrush or blender, or even the unity terrain editor, and export it to world machine to do your fancy erosion and terracing stuff.
You could also do the battlefield style gameplay zoning, where you just don’t let the player walk past a certain area, so he could never see the edges of the map.
But the more i add to my scene the leggier i get. all i added was grass trees and rocks.