Blocks that show machine activity without having to open the controller block.
For example, a display block that displays various variables, maybe configurable?
Or for more rustic machines some kind of primitive gauge display?
Or just lamps that glow or blink?

Also, machines that make some sort of noise make me happy for some reason. ^_^

To add to this, it could be neat if a recipe would be able to modify a state or something on a block within the structure, while it is running. Say you use a furnace in the structure, a recipe then could say "if this recipe is running (or this structure in general), set the furnace to the burning state".

I hope any of that made sense lol