Energy breakthrough making it cheaper - everything needs energy to function, that's probably an obvious pick.
Internet in rural places - access to internet is needed for everything: new jobs, entertainment, services, goods etc. If there'll be a way to access internet in rural places all over the world - it'll probably be one of the best companies (thinking starlink or something similar, since preferably you'll need low latency).
Content production or content delivery - people will always want entertainment, ie games, movies or streams. It is very far from ideal both in production and delivery. Solving it will give insane reach and insane amounts of money.
Reimagination of hiring and changing professions.
Probably another social network - people are not satisfied, someone has to try.
If you had asked the same question 10 years ago, no one would have guessed Netflix... because it wasn't in the S&P500 at the time.
Following a recession, all new investors will panic and withdraw all their money and keep it out for years. They didn't know the market could be down two years in a row. Boomers reach a phase in their lives where they are only taking money out of the stock market, and spending it on Chinese goods. Inflation picks up, and smart money moves from stocks to gold and then bonds. This combination will lead to one of the worst decades for stocks since the great depression. 2028 and 2029 sees money starting to go back into the market. But the decade as a whole sees low gains.
But even in a recession, people still treat themselves by eating out at McDonalds and their new $5 Keto menu.