Financial experts from Warren Buffett on down often urge retail investors to own a portfolio spread across the broad U.S. stock market. And if you own a fund that tracks the S&P 500, you more or less have just that — a who's who of the largest stocks available on U.S. exchanges.

But what happens when a new bombshell enters the proverbial villa?

Index fund investors may be pondering that exact question, given the news that SpaceX CEO Elon Musk said he's considering a tie-up with Tesla . When SpaceX reported its third-quarter financial results last week, Musk said on the earnings call that he'd consider a combination with the Austin, Texas-based electric car maker, in what would be the largest merger in history.

Adding fuel to this quandary, on Monday, AI firm Anthropic filed an S-1 with the Securities and Exchange Commission, signaling the company's intention to go public.

Both could be huge companies.…