In the case of a larger US retirement portfolio, you would add a few more asset classes:

-Domestic Stocks
-MSCI EAFE
-Emerging Markets
-Corporate Bonds
-Government Bonds
-International Bonds

These could be done with the following ETF’s:

-Domestic Stocks:

-IVV or SPY: Two very popular and low fee US ETF’s that track the S&P500, they are both traded on US markets

-MSCI Eafe:

EFA – Ishares ETF that tracks much of the developed world

-Emerging Markets:

VWO – Vanguard’s Emerging markets fund, traded on US markets

-Corporate Bonds

AGG – ETF that provides exposure to investment grade bond market
LQD – ETF that provides exposure to investment grade bond market

-Government Bonds

SHY – ETF that invests in short term US government bonds
TLT – ETF that invests in long term US government bonds
TIP – ETF that invests in inflation protected bonds

-International Bonds

EMLC – ETF that invests in international bonds

As you can see, there are many different possibilities even for a simple portfolio that only includes 5-6 ETF’s. I think the first step would be to select what you want in each of those asset classes. When you will combine that with your investor profile, you will get a very good idea of what to include in your portfolio. Here is the table again: