Of course, there is no way of knowing for sure what demographic changes will end up being the most important in the coming years. However, I would predict that the growing Hispanic population and the aging of the population in general will be the most important.
The growing Hispanic population is likely to have an impact on partisan politics. That is, as things are right now, this trend will help the Democratic Party and make it easier for that party to win elections in coming years. This is because the current policies and attitudes of the two parties make more Hispanics favor the Democrats. So, this trend will help the Democratic Party or it might, alternatively, cause the Republicans to change some of their policies to attract these voters.
The aging of the population is likely to have an impact on what the government has to do. Specifically, the aging population will force the government to make changes to the systems of Social Security and Medicare. It will need to do so because these systems might run out of money as we have more elderly people who rely on those programs and fewer younger workers to pay into them.
This is a great question. With the way the United States is going right now, our country is becoming more and more diverse. As mentioned in the above answer, as hispanics continue to increase in population, that will certainly lead to more Democratic votes come election time. Not only hispanics, but with the increase of people coming from around the globe, Asia, Europe and South America, the Democratic party's beliefs will also cater more towards those citizens. 20 years ago, a black president never seemed possible. However, thanks to where are country is moving and the diversity we continuously grow, a black president can thrive and succeed.
Aside from looking at a racial demographic however, I think another group to be on the look out for is the new tech industry. This new generation has already put a great deal of focus and attention on technological development. Companies like Google and Facebook are all digital and web based. Companies like these are constantly on the lookout for creative individuals with a desire to code, develop programs, and work exclusively on computers. It is a demographic that has propelled us into a more digital society and a more electronical business industry. At the rate at which ideas are developed and put into action, I fully expect to continue to see a lot of growth and improved performance in the field.