Additional water capacity to increase our food production.
Major civilisations such as the ancient Romans and Egyptians were built upon having adequate water supplies.
In a world of increased drought and severe weather, we could benefit from building additional desalination plants and/or stormwater harvesting to meeting the increased agricultural needs through population growth here and overseas. Population growth also impinges upon agricultural land, particularly overseas where land is limited. There will be increased demand for food and hardwood in the future, which we could capitalise upon.
Farmers have long held the belief that trees help bring rain. Increased forestation and irrigation would help cooling of the earth and also reduce global warming and assist with increasing rainfall.
The hard surfaces of our cities (rooftops, roads, pathways, etc.), our storm water drainage system (which currently runs into the sea) can be used to collect storm water. Suitable research into the most viable method of storm water harvesting, (such as pumping it into aqua ducts underground which could be later harvested by bores) could be undertaken.
Perhaps a combination of desalination and storm water harvesting could be used.
Picture source: http://www.waterencyclopedia.com/Hy-La/International-Cooperation.html