For the foreseeable future, which types of countries will see the greatest increase in population growth

Countries with relatively high populations, but lower population density have the highest potential to become highly populated.

Explanation:

Countries that already have relatively high populations obviously can sustain higher populations. Their cultures are likely ones that favour having many children, and the country is not so poor that families are able to grow large. Countries that have these features, and a lower population density have the highest potential to become highly populated because there is more land area for the people to spread out over and enlarge and create new cities.