The three leading causes of burden of disease in 2030 are projected to include HIV/AIDS, unipolar depressive disorders, and ischaemic heart disease in the baseline and pessimistic scenarios. Road traffic accidents are the fourth leading cause in the baseline scenario, and the third leading cause ahead of ischaemic heart disease in the optimistic scenario. Under the baseline scenario, HIV/AIDS becomes the leading cause of burden of disease in middle- and low-income countries by 2015. [see the full comparison of causes of death 2002-2030]
Projected life expectancy at birth increases in all regions, but there are still huge discrepancies based on income and other factors - for example women in high-income countries may expect to live to age 85, while men in Sub-Saharan Africa will have a life expectancy of less than 55.
• "The proportion of deaths due to noncommunicable disease is projected to rise from 59% in 2002 to 69% in 2030." That doesn't seem to be a very big change to me, so in 2030 we may have a similar rate of heart disease and cancer.
• "Global HIV/AIDS deaths are projected to rise from 2.8 million in 2002 to 6.5 million in 2030 under the baseline scenario, which assumes coverage with antiretroviral drugs reaches 80% by 2012." This is a great increase, particularly in parts of the globe already hard-hit by the diesase.
• "Total tobacco-attributable deaths are projected to rise from 5.4 million in 2005 to 6.4 million in 2015 and 8.3 million in 2030 under our baseline scenario. Tobacco is projected to kill 50% more people in 2015 than HIV/AIDS, and to be responsible for 10% of all deaths globally." So there is the big killer, unless human behavior changes appreciably (and future scenarios in which people don't take drugs of some sort seem implausible to me) or some way to counteract tobacco's effects.
Of course, unpredictable events are hard to factor into the projections. Will there be an infectious agent from space, alá The Andromeda Strain? Or perhaps an improvement as simple (yet with profound consequences) as sanitation*? That's where fiction comes in!
Paper Citation: Mathers CD, Loncar D (2006) Projections of Global Mortality and Burden of Disease from 2002 to 2030. PLoS Med 3(11): e442
* BBC article: Sanitation 'best medical advance', based on a survey by the British Medical Journal
(via Science to Life)