Hi Mark,
I would like to add that there is indeed an alternative asset class, which can add value to all mildly advanced retail investors, who understand correlations and Beta values: Simple pure trend following with managed futures.

The late Harvard finance professor John Lintner first co-created the Capital Asset Pricing Model (CAPM) in parallel to William Sharpe. In 1983, he also published the well known "Lintner Paper," formally titled “The Potential Role of Managed Commodity-Financial Futures Accounts (and/or Funds) in Portfolios of Stocks and Bonds.” This quote summarizes his tremendously beneficial research result:

“The combined portfolios of stocks (or stocks and bonds) after including judicious investments in appropriately selected sub-portfolios of investments in managed futures accounts (or funds) show substantially less risk at every possible level of expected return than portfolios of stock (or stocks and bonds) alone.”

This result is valid until today. Not in hindsight for a few lucky of these futures trading funds. But for decades on average of their active benchmark index SG Trend after costs of the 10 largest investable trend funds, selected at the end of each year for the next one. Therefore, this alternative asset class is one of the very few with true added value to retail investors.

Thus, it can be recommended to the benefit of them, interested in outperformance at least in risk adjusted terms, to diversify their equity ETF portfolio with a number of these funds. They should be included in the benchmark index or have a high correlation and beta to it.

As these funds are non-coorelated to equities on long-term average, and negatively correlated to them in financial crashes, they can substantially reduce portfolio drawdowns then without reducing return potential long-term. They require about the same effort as ETFs to select the ones with the right properties, and then buy-and-hold them.