As far as the laws of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality. -Albert Einstein
One option is the scenario approach to reality. Managers should stress a portfolio based on events that are possible but not actually found in the empirical data. Be ready for what has not yet occurred.
Second, managers should use a precautionary principle to portfolio construction. Build a portfolio that can survive any extreme event. A precautionary principal can take the form of a barbell between low risk and high risk portfolios. The high risk portfolio can potentially take a devastating hit, but still leave the overall portfolio able to survive to some predetermined loss.
Finally, a core principle can be to never lever a portfolio beyond a set level especially in a low volatility world. A key problem was with the target volatility crowd is that they continued to add positions to maintain volatility targets in a low volatility environment. A max leverage principle would have capped the leverage regard of target volatility levels. Volatility may stay below the target level but leverage caps are more important.