With various plugins available on the marketplace, such as Directional and Planet Gravity by Tefel, Voxel Plugin Pro by Phyronnaz, and also the new sky atmosphere system built in to UE4, things are opening up to allow for spherical worlds in Unreal Engine. One feature sorely lacking from these things is dynamic navmesh generation for spherical worlds. If there was a way to generate navmesh dynamically based of the current up vector (center to surface of sphere), it would add the ability to add AI with the full functionality of all the AI features in Unreal, without having to resort to a hacky approach to get NPCs moving. I doubt this is a small task, but I think it would open things up more and let us break free from being constrained to global z.