Probably the most complex structure ever built by humans which is slowly approaching its retirement... I wonder if we'll replace it with another constantly manned outpost in space. Bigelow Commercial Space Station seems to be the closest replacement for ISS; after all they have been testing an experimental module for over two years now and it's expected to stay attached to ISS for two more years at least. Axiom Space are also working on a similar autonomous space station. Once viable and affordable launch vehicles are readily available (SpaceX and Blue Origin are developing reusable rocket ships), there will be a big breakthrough especially when it comes to commercial space flight and commercial space stations. However, there are by far more interesting concepts and ideas being developed as the potential "next step". There are plans for a Deep Space Gateway, a lunar orbit space station which is intended to be used a jumping point to the exploration of the solar system. Going to the Moon itself is another big one, with various governments and private agencies and corporations planning to do that in the near future, and it's possible that we'll have a permanent Moon base relatively soon. Asteroid mining. Manned missions to Mars. I think we're entering the new golden age of Space Exploration at last; along with the usual suspects (i.e. USA, Europe, Russia), other countries have high ambitions and plans in development too, especially China and India. So exciting.