What will Tokyo be like a hundred years from now? Three scenarios come to mind.
An Eco Edo
An optimistic scenario is that Tokyo will be an "eco-city" and world-class research center. It could become the robotic capital of the world and leader in many high-tech fields. For this to happen, education will need to improve. Instead of fostering mediocrity, schools could promote advanced creativity and human potential. Robotics and space exploration could be key industries and the city's skyline would have several buildings over 800 meters high. These structures will contain carbon nanotube composites and macro-crystals stronger than steel. A space elevator will be located near the former Narita Airport. Substantial portions of Japan's population will be heading to space colonies and large portions of Earth will be wildlife refuges.
Tombstone Tokyo
A pessimistic scenario is to envision Tokyo in the aftermath of WWIII. As the population soars and competition for the planet's resources heats up, the possibility of a global conflict must be acknowledged. If testosterone-poisoned tyrants control the government and industry leaders come to believe armed conflict might be profitable, that scenario could take place. Moreover, as our knowledge of DNA expands it'll become increasingly easy to design bio-weapons to wipe out vast populations. The samurai consciousness present in many Japanese
could foster a kind of xenophobic nationalism and autocratic way of thinking. Full scale biological and/or nuclear war could easily eliminate humans from this planet. If humans do survive another war, most scenarios are dystopic.
Stalemate City
In between these extremes, a hundred years from now Tokyo will likely be a mega-city struggling to fulfill a patchwork of conflicting agendas. One segment of its population will live in unsurpassed wealth while other segments subsist in poverty. Many human rights will likely erode in the name of "security" and the general population will be kept more or less content through sophisticated brain-washing and social control. Surveillance cameras will be omnipresent. "Democracy" will seem like a naïve notion because the imbalances in wealth will give rise to marked imbalances in power. There might appear to be elections, but these will be orchestrated frauds with little validity. Each citizen will have an identity implant and the government will keep close tabs on all persons at all times to promote "safety".
Nearly 30% of the people living in Tokyo-Yokohama will be non-Japanese. Many will be health care workers from Asia. However, wealthy investment bankers and technology specialists will also abound.
Large portions of Tokyo will be rebuilt following a recent major earthquake that destroyed much of the old city. Wealthy people will in special "safe" parts of the capital with enhanced security, but poor people will live in tiny cubicles. Due to global warming, parts of Tokyo will be under water and summer temperatures will soar to around 50°C.
In short, Tokyo will be a city full of contradictions – a place that people both love and hate. Many of the metropolitan areas across the planet will be facing the same problems as Tokyo. To a large degree, the city will be choking from its own toxic wastes and bureaucratic inefficiency. Many citizens will enjoy relatively clean, comfortable, and sanitized conditions. However, a portion will likely subsist as disposable techno-serfs: drones in a society in where social ennui and isolation are pervasive. On the whole, Neo-Tokyoites will be docile and obedient for reasons that they are only partly aware of.
An optimistic scenario is that Tokyo will be an "eco-city" and world-class research center. It could become the robotic capital of the world and leader in many high-tech fields. For this to happen, education will need to improve. Instead of fostering mediocrity, schools could promote advanced creativity and human potential. Robotics and space exploration could be key industries and the city's skyline would have several buildings over 800 meters high. These structures will contain carbon nanotube composites and macro-crystals stronger than steel. A space elevator will be located near the former Narita Airport. Substantial portions of Japan's population will be heading to space colonies and large portions of Earth will be wildlife refuges. 
