Having played Medieval Engineers for a long time, I've found that the worlds feel very empty upon creation, and often for long periods after. This is an issue that I've heard several other people talk about, as well, and I've developed a suggestion that might help resolve it. It is a similar idea to the Barbarian Camps and Types suggestion, made by user József Szabó, but with a number of differences.
I believe that stone and/or wooden ruins that were procedurally added upon world creation would be a very nice addition to the game. Seeing old, abandoned structures strewn about the countryside, I feel, would really add to the character and implied history of the landscape, and create interesting locales to interact with in a number of ways.
For starters, I believe that it would be unlikely for a group of desperate people one would refer to as "barbarians" to be able to construct anything permanent by themselves. Stone towers and structures are large and require a lot of resources and time to build; something a roaming band of misfits wouldn't be able to gather without essentially forming its own small kingdom. Something that would be believable, however, is if there were a number of ruined stone structures around, that barbarians could sometimes move into and fix up using available materials (wood and cloth, that sort of things). From there, barbarian parties could periodically spawn and attack players and their settlements within a certain radius, creating an incentive to deal with the threat and mount an attack/straight-up siege.
Secondly, they could form a foundation for players who don't want to build an entire castle town from the bottom up. It takes a long time to perform all the research and gather the resources required to build large stone structures, and this can feel daunting to those can't afford to play long sessions at a time. Having something of a starting shape and a partial structure could really make building a base much more fun for those people.
And finally, it would be a fun way to showcase and encourage community efforts, by adding community-created structures to the pool of possible ruins.
I believe that this would work together quite well with József's suggestion; but if continually spawning new structures/changing existing ones proves to be too resource-intensive (be it to create the code for it or in the hardware load sense), just spawning a number of empty structures in upon world creation would only be a one-time use of resources and it could use mostly the same code that Space Engineers' pirate bases use.
Thank you for your time, and have a nice day!