So what about 100 cities?
Apparently, my GTX 1080 was kinda struggling with rendering 1 million trees at the same time when looking at the world from above, so it was time for some more optimizations.
I set a new goal for this project: make a huge world bigger than the skybox, with over 1 million trees, over 100 cities connected by roads, and have it optimized enough to run even on mobile 
Here is the result so far: https://www.fortnite.com/@redcubex/4662-3510-3563
This was possible using UE5 tools that aren’t available in UEFN yet, so it required a lot of UE5 - UEFN migration and some custom UEFN Python tools.
Here is a quick rundown of the pipeline:
The terrain
The landscape is made using Mesh Terrain, which allows me to easily partition the world and add detail only where it’s needed.
For example, I can easily flatten the areas where the cities are, even after changing their size or location.
The downside is that Mesh Terrain is still experimental, and some features, like Spline Remesh, which I wanted to use for the roads, are still a bit buggy, so I couldn’t really use them.
The landscape is around 81 km². Initially it was 100 km², but that was kinda big. It’s split into a 5×5 grid of meshes, with only around 3k triangles each right now, so I can load the entire terrain at the same time without much memory usage.
Cities and roads
The cities, roads, and trees are all placed using PCG. This took me some time to figure out, but PCG has been a lot of fun to learn and use.
The cities are quite simple right now, mostly some blocks with roads. This is pretty easy to do with the PCGEx plugin. I even found another plugin called Watabou that can generate some cool cities, although I didn’t end up using it.
Unfortunately, PCGEx has some issues right now where some of the end connections don’t work properly. The engine seems to treat them as start connections, which means you can’t connect them normally. With some workarounds, though, I managed to get the cities and roads working.
The roads are still my biggest issue. Since the terrain is uneven and I want to create intersections, different road widths, and other variations, I’m still trying to figure out the right solution for this part.
One million trees
Now, this was probably the biggest optimization challenge.
Initially, I managed to get all 1 million trees loaded and fit them within the memory limits, but the game wasn’t loading properly, and there was basically no chance of getting it to load on mobile.
UE5 has a plugin for generating impostors, basically very cheap representations of meshes designed for distant objects, which is perfect for trees.
It was a bit of a hassle to export the results and make them work properly in UEFN, but after some tinkering I managed to get the billboard version of the impostors working, and they work really well, especially on mobile.
The trees themselves are also generated using PCG, but they’re partitioned into around 400 actors so they can take advantage of the engine’s World Partition system.
Conclusion
Overall, I’d say making really big worlds in UEFN is definitely possible.
That said, I’d probably recommend waiting for Mesh Terrain and PCG to eventually make their way into UEFN. Otherwise, you have to go a pretty long way around: creating everything in UE5, exporting/migrating it into UEFN, and then making custom Python tools to adjust settings and fix things that don’t survive the migration properly.
But it works — 100+ cities, 1 million+ trees, 81 km², and running on mobile.
