I've been looking at printing figures (people, animals, etc) but there's not many models available in HO scale. Of course the great thing about this hobby is it's very easy to change the scale of an object before you print it.
A great source for models is in 28mm minis (used a lot for D&D type gaming, I think?). If anyone else goes down this path, just be aware that 70% is the magic scale number you're looking for. Take a 28mm figure and scale it down to 70%, and you'll be within a decimal point of the size needed for HO.
Actually *printing* those models is something else entirely. At HO scale a standing person is only about 15mm tall, so you need pretty fine detail. I'm going to be working with a 0.2mm nozzle and 0.04mm layer heights. Initial testing showed that even a slight bump in the belt can destroy a whole head. Next time I try it I'm going to slow the print speed way down. I'm hoping to get my prints good enough to see facial features.
As for printing buildings, I do have one link saved that might provide a search point for you: https://www.thingiverse.com/thing:2981217
This is just a flat print of a stone wall that you could cut out for models, and I wouldn't be surprised if there were other similar textures to be found.