Player built structures.
This was a ton of work. Just getting enemy agents to manuever around them was hard. Getting structures to combine, break apart, and rejoin was another challenge.
Then adding in the fact that having a building on the other side of the planet had some performance impacts so I've added them to the Grid Manager. That means they now return all their parts back to their object pools for reuse when not in an active area or being interacted with by agents. Which that itself raised new challenges. If a structure stashes itself away... how does it's factory keep working. So all machines needed a rework so they can hand off their functionality to a simulation layer rather than require physical game objects just to keep working.
Then as a bonus I wanted to use the structures system to prepopulate the game with structures. This way I could create a new underground starting area. Which will improve the first few seconds of gameplay and give me a way to briefly force the player through skill checks as a light tutorial.
This meant engineering a whole bunch of engine systems to support artistic workflows in the Unity engine. So that meant I've now created two structure creation systems. One for developers to be able to work efficiently and one for the player to use in game. The developer system is more powerful and flexible, while the player system is simpler and more user-friendly, but both require a full suite of tools so anything a dev makes, a player can make too.