The introduction of Object Culling Volume was very important for improving performance in the game. Thinking about how you could improve it even further, a very simple, but actually very important idea came to mind for large-scale maps that have both surface and underground content. The idea is to create a volume that would work like an Object Culling Volume, but when you enter it, objects that are outside of that volume would be unloaded. This can greatly help in optimising caves, basements and other places where there is no access to open space.
Actually good suggestion
The only thing I worry about would be reloading everything when leaving the volume. This could be very problematic for certain things like a underground segment under a city. A potential solution would be to have a “feathering” option to keep things in a modifiable radius outside the volume loaded in as normal. And also to make it harder for players lagging their game by crossing the volume’s boundry, a brief timer could be added to prevent mass rapid loading and unloading over and over.