Tiger-Backed Robinhood Rival Launches DIY Indexing With a Twist

What if you wanted to build an index betting on companies traveling to Mars, firms with the most social-media followers, or those with CEOs over six feet tall?

Ideas like these live outside the bounds of traditional index construction on Wall Street — too niche, too speculative. But a new offering from investing platform Public Holdings Inc. is testing whether that kind of demand exists among everyday investors.

Called Generated Assets, it lets users create and backtest custom indexes based on a slew of themes. Soon, they’ll be able to invest in those portfolios directly — and even publish the ...