It turns out that regular, index-based retirement portfolios are in fact enough to help you retire comfortably. However, once you've developed a solid core of globally diversified index funds, adding satellite positions designed for a specific purpose can create value. Here, we'll look at four ETFs that might make your retirement portfolio a bit more interesting. 

1. Schwab Dividend Equity ETF 

The Schwab US Dividend Equity ETF (SCHD -0.86%) is an exchange-traded fund that comprises U.S.-based companies that pay consistent and meaningful dividends. Stable dividends are desperately sought after by many retirees and pre-retirees as reliable income. Moreover, dividends are important because you can develop a stream of passive income without needing to sell any underlying shares of your investment. 

A fund like this is best held in a tax-deferred retirement account like a 401(k) or pre-tax IRA. The reasoning is simple: When you receive dividends in a tax-deferred account, you won't pay tax upon receipt. You can receive dividends in a tax-deferred account for many years without paying a dime in taxes; you'll only owe the IRS any money when you withdraw money from the account. And this may not occur until you're age 72 and required minimum distributions begin. 

2. Vanguard Dividend Appreciation ETF

The Vanguard Dividend Appreciation ETF (VIG -0.84%) doesn't necessarily include companies that pay high dividends, but instead it includes companies that have long histories of increasing their dividends. Companies that fit into this category have also typically experienced sustained increases in their share prices over time. This means that the value of holding such a fund comes to the investor in the form of total return: price appreciation plus dividend income. 

This fund is best suited for a taxable brokerage account for a number of reasons. Companies with rapidly increasing share prices are best held in taxable accounts because you're able to take advantage of favorable long-term capital gains rates, which can range anywhere from 0% to 23.8% of realized investment gains. These tax rates pale in comparison to ordinary income rates, which will apply to 401(k)s and pre-tax IRAs. Ordinary tax rates, when including state and local taxes, can reach over 50% if you live in New York or California. 

Couple evaluating retirement plan.

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3. Vanguard Small Cap Value ETF

The Vanguard Small Cap Value ETF (VBR -1.20%) can be a return-boosting fund for a number of reasons. First, there is some evidence that small-cap companies, particularly those with "value" characteristics, have greater returns than their large-cap adversaries when held over the course of many years. 

Famed professors Eugene F. Fama and Kenneth R. French found that a "small cap value premium" exists, at least when examining data from 1963 to 1990. There is a commonly held belief that such a premium still exists, but some naysayers posit that small companies receive less widespread coverage and are therefore subject to more frequent mispricing. 

Regardless, a small-cap value ETF can make sense in many circumstances, especially as a long-term hold in a tax-exempt account like a Roth IRA. Earnings and distributions from a Roth IRA are entirely tax-exempt, so it's a good idea to place funds with high expected returns in a Roth -- you'll never pay taxes on this money once it's been deposited and invested, so it's prime real estate for high-growth holdings.

4. Amplify Transformational Data Sharing ETF 

Of the funds mentioned here, this is the only one with an expense ratio that merits caution, at 0.71%. The Amplify Transformational Data Sharing ETF (BLOK -4.12%) is meant for investors who are interested in blockchain technology but may not have the stomach for holding cryptocurrency directly. This is a compelling play for any investor -- not just for retirees -- but it does come at increased cost.

The perils of investing in Bitcoin (BTC -1.07%) and other cryptocurrencies are well documented: Investors are often scared away due to wild price swings and the uncertainty associated with cryptocurrencies' mainstream adoption going forward. This particular fund allows you to invest in companies that are building the underlying technology for cryptocurrencies, which can help diversify risk while still allowing for substantial return potential. 

The bulk of return from this fund will come from price appreciation, so it's perfectly suited for a taxable account or Roth IRA. It also provides a psychological cushion for those that want a crypto-related investment but don't want to have to choose among the hundreds of coins currently on the market. 

Allocate carefully and hold indefinitely

When it comes to building a flexible and robust retirement portfolio, you'll need to invest in many different asset classes, diversified on a global level. You'll also need to make sure the right investments are held in the right accounts through careful tax-efficient asset placement. Check out any of these ETFs to make your retirement portfolio a little more exciting and to hopefully add a fair amount of return.