Nexus City is an unreleased world that would have eventually surrounded Nexus Tower. Nexus City is one of the first worlds designed for LEGO Universe, with concepts of it dating back to 2007. The Monorail in YouReeka would have let players travel to Nexus City, but it is unknown how players would have reached it after YouReeka's redesign into Nimbus Station. Despite being mentioned on the LEGO Universe website and appearing in several pieces of concept art even after LEGO Universe's release, the city was never constructed and opened in-game due to the closure of LEGO Universe. Figmentia, one of the LEGO Universe community personnel, confirmed that plans for the world were scrapped at some point prior to the closure of the game. (Source?)
The Maelstrom has blown Planet Crux into thousands of worlds before forming into a swirling black hole of chaos energy! Opposite the dark menace’s center sits Nexus City. Nexus Tower rises from the city’s center, glowing with light from the last shard of Pure Imagination! Other large planet chunks drift around Nexus City. As a LEGO Universe Minifigure, you can explore in the festive Avant Gardens, then hop in your custom LEGO spaceship and fly to new worlds of creative fun! You might set off on pirate-themed adventures in Gnarled Forest, join a Nexus Forcefaction in Nimbus Station, or explore ancient ninja ways in Forbidden Valley! Drifting among the larger worlds are thousands upon thousands of smaller ones. Now, you can settle on one of these worlds and use your creativity to help push back the Maelstrom! Get inspired flying between your friends’ properties, then head for your own LEGO Universe world. There, you can build anything you like— LEGO brick by brick— and bring your creations to life!