So this is a couple suggestions rolled into one.
I have played several open world/builder games that seem to plateau quickly once you reach a certain level. 7daystodie, timber and stone, etc… Attacks on your base become insignificant, OR they are scaled to get worse and worse until you die. In both scenarios your game becomes linear and stale after a while. One solution to this in my opinion is to mix up enemy attacks/spawns with random points of interest spawns and world events. These could include neutral towns settling nearby, enemy outposts being built, cults of evil hearthling settlements and monuments, evil dwarf mines, etc etc. People are more creative than I am, but you get the idea.
I think in addition to wave after wave of attackers, these random events/spawns would force you to leave your base and explore. Maybe you would then need to build outposts, or trade, or attack, or loot outside of your walls.
These POI spawns level/difficulty should go up with your own wealth and with the average level of your hearthlings. Maybe some of them spawn ahead of where you are, to add some future difficulty / goals. They wont attack you, but after exploring you are forced to hold off attacking them.
Hope that was clear as mud! 