TOKYO — From the moment in September when Tokyo won the tight race to host the 2020 Summer Olympics, locals have been unleashing ideas — some far-fetched, others tantalizingly imaginable — of what the city will offer the more than 10 million spectators expected to attend the Games.

Self-driving cars are a given, auto enthusiasts say, perhaps reserved to whiz V.I.P.’s on designated lanes through the traffic-choked capital. To help hoi polloi navigate the city, buses powered by fuel cells will be commonplace, the futurists say. Older people in the graying country might wear robot suits to get around. Next-generation translation services, streamed through wearable technology, will help locals communicate with foreign guests. And perhaps there will be some climate engineering to ease the heat and wretched humidity of the Tokyo summer.

But there are also worries about the future. Tokyo, with 13 million people, is one of the world’s most disaster-prone cities — earthquakes, typhoons, tsunamis and storm surges. Experts worry about the effects shifting weather patterns will have on the Tokyo Bay section where many of its new Olympics venues will be built.