The key to outsized investment returns is not about what's happening today, but instead, what's going to happen next. In terms of the emerging markets, that means trying to figure out the next economy that will explode with growth. To find the next big thing, we need to search the world over, and learn how to invest in it today.

If you've heard of it already, it's not the next big thing
Based on recent history, a few usual suspects probably jump to the top of your mind. China is the most obvious one.

The Chinese economy is still growing at a rapid clip, but that growth is beginning to taper. Chinese investors are actually concerned with the country's projected 6.5% to 7% growth this year, a rate that would represent a 1% to 1.5% reduction from just a year ago. Despite the massive size of the Chinese market and the huge opportunity it still presents, China is already too mature to be considered the next big thing.

We can go ahead and eliminate the rest of the so-called BRIC nations, as well -- Brazil, Russia, and India. Growth remains brisk in India, with GDP estimated to rise around 7.5% this year, while Brazil and Russia have both faced serious economic problems during the last few years.

Regardless of the current condition of these countries, we'll omit each from this discussion to avoid rehashing the already well-known BRIC nations. To find a true hidden gem, we'll have to dig a little deeper than these somewhat obvious choices.

Five potential emerging markets for the next 20 years
To help guide the search, we'll divide the world into five regions of potential emerging growth: emerging Europe, Latin America, the Middle East and Northern Africa, Sub-Saharan Africa, and the confederation of states of the former Soviet Union. We'll exclude emerging Asia because of the aformentioned influence of China and India.

We'll use current and projected population and per-capita production figures to assess the potential upside of each region. Economies can grow by having huge numbers of people producing a little, or by having each person produce more individually. Multiply the total population and individual production to define the full size of each economy.

We can immediately eliminate emerging Europe, constituted by Turkey and the eastern European states, and the former Soviet states, simply based on population. These two regions have markedly smaller and slower-growing populations than the other regions, a fact that effectively puts a ceiling on their absolute growth. The per-capita growth required to compensate for their smaller numbers simply isn't a reasonable possibility.

We also can eliminate Latin America, albeit for slightly different reasons. Latin America, which includes Mexico and the Caribbean, has ample population to support an economic boom, but economists expect per-capita GDP growth to drop by nearly 50% during the next five years. That decline in individual production eliminates Latin America as the next big thing, despite its large and growing population.

That leaves the Middle East and Northern Africa and Sub-Saharan Africa to consider. Both regions have comparable working-age populations that are expected to grow rapidly during the next 10 to 20 years. Both regions are expected to maintain or grow per-capita GDP, meaning that their growing populations will work more productively, driving an even larger increase to GDP in absolute terms. 

There's one not-yet-mentioned factor that differentiates the two: the current account balance of Sub-Saharan Africa.

The IMF reports that Sub-Saharan Africa had a negative account balance from 2010-2014, and expects that gap to grow to nearly 4% of GDP from 2015 to 2020. That means that the countries of the region are supporting themselves -- and their economic growth -- with foreign money instead of domestic production. This puts Sub-Saharan Africa in a precarious financial position if that foreign money were to dry up. 

Therefore, based on each region's demographic trends, projected GDP growth in absolute and per capita terms, and their current fiscal situations, we can justifiably predict that the economies of the Middle East and Northern Africa have the best chance of becoming the next best thing.

For investors in the U.S., the easiest way to invest in this region is through an ETF focused on the region. The SPDR S&P Emerging Middle East and Africa ETF (NYSEMKT:GAF) is a popular option. This fund charges a 0.34% management fee, and has annual operating expenses of just 0.30%.

Emerging markets are fraught with risk, and this is no exception
It's an unfortunate reality that the low per-capita GDP that defines an emerging market also makes it more likely to have civil unrest, unstable political structures, and oppressive economic dynamics. The Middle East and Northern Africa are not exempt from these realities. The Arab Spring led to the overthrow of several middle eastern governments in 2010 and 2011. Even today, the region is rife with violence and even war. 

Piling risk on top of risk, the projections used in this discussion for per-capita GDP growth and broader economic performance could prove to be off base, even from the highly respected analysts at the IMF and elsewhere.

Even without a crystal ball, the data points to the Middle East and Northern Africa. Nothing's certain, but based on population factors and projected economic trends, the region warrants a closer look.