(Fortune Magazine) -- If you were a typical American investor on Dec. 31, 1999, chances are your portfolio was loaded up with stocks -- U.S. stocks, for the most part. And scorching tech stocks most of all. Why not? The S&P 500 index had swaggered its way to a 432% total return during the '90s. Sure, there were a few zealots on the fringe proclaiming the virtues of commodities and emerging markets, but who was listening?

We all know how that turned out. Even as the S&P sagged by 11% through the '00s, gold quadrupled, oil took off, and you had to listen in agony as a work colleague bragged about the killing he'd made in a Peruvian mining stock.

Is the paradigm about to shift again? We asked four of the market's best minds to preview the next 10 years. Their answers mostly point to another sobering decade in the U.S. Opportunities still exist -- you just may need to look a little further to find them.

Robert Arnott 
Research Affiliates chief; plots strategies used to manage $43 billion

There are three challenges that will shape the landscape in the decade ahead. Fiscal discipline has been largely lacking at all levels, creating a debt burden far beyond historical precedent. If you factor in Social Security, Medicare, state and local debt, and what Fannie Mae and Freddie Mac owe, our total public debt is now at 141% of GDP. Add in household and corporate debt, and the unfunded portion of entitlement programs, and it's 840% of GDP. Yikes.

The second headwind is the fact that markets are expensive by historical standards.

The third is demographics: In 2002 the U.S. was adding 10 new working-age people for every new senior citizen. By 2023 that flips to 10 new retiree wannabes for each new working-age person. Retirees will be selling assets to a shrinking pool of buyers. So save aggressively and play defense.

While the developed world has huge debt and demographic problems, many emerging markets have younger populations and foreign reserves. After the rally in 2009, this is not a "buy now" recommendation. Still, a case can be made to invest significantly more in emerging markets, especially during one of their periodic plunges. U.S. inflation and a declining dollar will add to their relative attractiveness.

Inflation protection will be priced at a premium. Give inflation-linked bonds and commodities more than token allocations. There are always interesting investment opportunities. Choose wisely.

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