Posts like these seem a bit silly to me because it's incredibly easy to spend billions of dollars as soon as you get away from a consumer mindset. The value in a billion dollars comes from being able to shape the future in whichever direction you want. I'm not interested in spending money just to make my own life marginally more comfortable, but in elevating the quality of life and overall experience of everyone around me.
Here's a few random frustrations I have:
Most modern hardware appliances are not easily repaired or hackable. I'd love to manufacture and sell open hardware appliances which prioritize repairability and maintainability, including sharing the CAD models and opening up the firmware.
Despite the years of effort that have gone into the Linux Desktop Experience, it still often lacks polish in various areas. You could afford to hire world class engineers and designers to fix up every minor annoyance and really provide the most deluxe desktop experience possible without compromising on the slightest detail. Not only that, you could contract companies to add Linux support for any essential tools and applications which aren't already supported.
And that's not even getting into the ability to fund the creation of really outstanding media. Most modern kid's entertainment treats them like morons while slapping them in the face with basic lessons. You could create some truly delightful kid shows without having to skimp on any aspect, and really lay the foundations for creating a brighter future. Embed lessons of every major topic as part of the show without being hamfisted about it, and when they start to encounter those challenging topics in school they will have some foundational models on which to build upon. A basic example: you can teach a kid the fundamentals of calculus from an intuitive perspective, and when they actually learn proper calculus in school it'll be much easier to ramp up.
Heck, you could fund the modernization of a ton of college level educational content with enough money. Buy the rights to any important textbooks, rewrite as needed, then make them freely available. Hire a team of world class artists, animators, and builders to help create supporting materials / content that cover any topic. Pair that with world class educators and experts. You put that all together and create the most powerful repository of high quality educational content that the world has ever seen. By doing this you're laying the foundations for the development of future generations and setting them up for success!
Those are just some quick thoughts which I'm willing to write up here... If I thought about it longer I'd probably be able to come up with more significant quality of life improvements that could be spread out if someone was willing to spend a few billion dollars into making them into a reality.
Oh here's a final quick one: funding maker spaces across the country. It's not clear how much potential could be unlocked if we had widely available maker spaces where people can ask for help with their projects and ideas. Sometimes all it takes is having someone who can point you to the right tools or people.