Hi @DonaldFaulknor.
In my opinion, there are several factors. The first would be single player or multiplayer. Things get tricky with multiplayer see the big world documentation. Another factor to consider is the construction of the game world environment using unrealengine landscapes or custom solutions like voxels. Is there a significant amount of Architecture? You may find yourself creating custom occlusion methods and transportal systems to optimize. Its a lot of work to create a large environment, so you may find yourself seeking procedural generation systems and techniques (which is what Im doing as a solo dev).
Good Luck.