The following algorithm is based on the German Faulbär-Strategie introduced by Christopher Klein, which I roughly translated as "Lazy-Ass-Strategy".  The one rule is ridiculously simple:

Once a year, invest 100% in the ETF with the worst anualized return from a list of country-index ETFs.

The reasons why this strategy might work, are open for discussion. One reason could be some kind of a mean-reversion rule, another one could be administrative interventions such as quantitative easing. 

The one parameter for the simplest version of this strategy is the static universe, which should obviously only contain ETFs of relatively stable countries. Other parameters such as trend filters may be added, to reduce the maximum drawdown.