There's a new sheriff in town. Previously YC treated some of these things, especially "AI" (whatever that means) as third rails. They've funded an extremely broad range of companies, but if you go back and check their past investments most of the areas listed in this RFS are underrepresented. Infrastructure, food and water, biotech, robotics...YC could be reaching much higher in terms of ambition than it has in the past. This will also mean more failures, but sama is smart and tough enough to hold on through it.
But realistically, how could YC get involved with early stage startups in most of these categories? Examples like SpaceX and Tesla don't have minimum viable products. You can't build part of an engine in three months and then show it off on demo day. Creating solutions for these problems at the most rudimentary level will require months if not years of effort and significant financial backing. It's hard for two guys in a garage to build a prototype 10X battery or come up with innovative biotechnology. In order to get in on these ideas early enough for it to make sense, YC might have to start throwing caution to the wind and accept founders who have only drawings and computer models. It's just so difficult for a small group of people to have a noticeable effect on problems on this scale.
The best way to get involved with the next world changing idea might be to choose capable and intelligent people and say "I'm willing to bet on anything that you do, even if you're not sure what it is or if it changes completely tomorrow". I think YC already has a process for accepting single founders without concrete ideas and they could expand on that. Or start making changes to attract founders whose companies are farther along in their development. No matter what they do, setting big goals usually doesn't hurt. I hope that the YCombinator organization is willing to make some of the changes necessary to move closer to them.