This year could well go down in stock market history as the year of the mega-IPO. Three of the world's biggest start-ups are lining up to go public this year, and investors have only just begun to consider the implications.

For example, SpaceX is seeking a valuation of $2 trillion ahead of its IPO, according to numerous reports. The company has confidentially filed for its IPO, which could kick off as early as next month. Artificial intelligence (AI) start-up Anthropic is in talks to raise capital at a $900 billion valuation, ahead of a public offering contemplated for later this year. The company's AI rival, OpenAI, was recently valued at $852 billion, ahead of a planned IPO later this year.

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With a combined value of nearly $4 trillion, Wall Street has begun to grapple with the enormity of these events. S&P Dow Jones Indices is considering revising the rules governing how companies join the S&P 500 (^GSPC +0.59%). If adopted, these changes would have significant implications for SpaceX, Anthropic, and OpenAI, allowing them to join the benchmark index much sooner.

How things stand now

The S&P 500 is widely regarded as the best overall gauge of the U.S. stock market and is composed of the 500 largest publicly traded companies in the country. To be considered for admission to the index, a company must meet the following criteria:

• Be a U.S.-based company
• Have a market cap of at least $22.7 billion
• Be highly liquid
• At least 50% of its outstanding shares must be available for trading
• Must be profitable based on generally accepted accounting principles (GAAP) in the most recent quarter
• Must be profitable over the preceding four quarters combined
• Be a publicly traded company for at least 12 months

It's that last rule that could be suspended for so-called "mega-cap" companies, generally defined as those with a market cap of at least $200 billion. Under the proposed change, recent IPO stocks could be added to the index as soon as six months after their debut, down from the current requirement of 12 months.

"MegaCap IPOs have the potential to achieve immediate and material investor ownership, trading liquidity, and market relevance," S&P Dow Jones Indices said. It went on to say that the current index rules could "prevent such IPOs' timely index inclusion and impact the overall index's effectiveness as a benchmark."

This would mark a stark about-face for the storied index. While there's a "fast-track" exception for entry into the S&P's Total Market Index, there is no such exception for inclusion in the S&P 500. In its written rules, S&P Global states, "IPOs should be traded on an eligible exchange for at least 12 months before being considered for addition to an index. There is no IPO fast-track entry allowed."

If the new standard is adopted, it could have significant implications for retail investors. For example, exchange-traded funds and other funds that track the S&P 500 would be forced to buy the newly minted stocks once they are added to the index. That might not seem like a big deal, but estimates suggest approximately $24 trillion is invested in S&P 500-tracking funds, according to Bloomberg.

The proposed rule change is still being considered. S&P Dow Jones Indices is soliciting comments until May 28, and changes wouldn't be implemented until June 8.

Not coincidentally, the new rules could be in place before the SpaceX IPO, expected in late June.