Big themes?
* Housing retirees. The whole spectrum from independent to low care to high care. All income levels. Will there be an need (possibly forced through economics) for intermediate style accommodation (As America does with its trailer parks)?
* Housing service industry workers. Eg the coffee servers, aged care workers, cleaners, security guards, retail workers that a CBD area needs to run. Need to be within commuting distances of the big cities as these people can't telecommute.
* Age care, health, tax and immigration. Ageing of the population will cause increased demand which will need to be paid for (large parts through taxing others). Ways of doing it cheaper will be sought by governments. Labour costs is a large component of that and if there are immigrants who are willing to work for less it will be difficult to resist bringing in 'guest workers' (ie 457s but for a wider range of jobs). Immigrants will thus be looking after some who have opposed immigration for most of their lives. Trade unions will continue their decline.
* Managing social diversity - the disappearing middle. The population that supports libertarian attitudes on matters like gender, race, sexuality is increasing. So is the population who support conservative, often religiously-based attitudes (the latter due to immigration from some areas). Can we all get along and how?
* Resources, including infrastructure and energy.
* Reduced productivity of land, including that caused by salinity and climate change.
* Urban settlement. If adding a few people each year the cheapest way to do it is add people on the fringes of existing cities. Medium density development is stymied by existing residents of 'blue chip suburbs' resisting change, pushing developers to build high-rises near the CBD or houses on the fringes.
But cities have diseconomies of scale. Eg a small city needs no freeways nor railways. Larger cities have freeways and (mostly surface) suburban railways. Both the latter become inefficient for megacities and expensive tunnelling will be required. Either for metros within cities or bullet trains for interurban travel.
But longer term we are perhaps better to have 10 cities of 3 million than 3 cities of 10 million. Successful decentralisation though is rare - Gold Coast and Canberra have overtaken Hobart but none of the five mainland capitals.
The problem is what's most expedient for the shorter term and what's best for the longer term is different and most of the future is spent unpicking the errors of the former.