My megaproject ideas are mostly pretty standard. I'd build a high speed rail network across North America, and build and expand metro and regional rail systems in and around every city. I'd turn all cities and suburbs into fifteen-minute cities. I'd decommodify housing, and build ten million units of public/social/non-market housing, mostly three bedroom units. I'd link those last three policies together by building TODs around the new Metro and rail stops. And I'd build bicycle networks in every town and city and connect them to the TODs. I'd build bridges and walkways across skyscrapers. I'd put a bidet in every American toilet (uses less water than toilet paper apart from being more comfortable). Fiber internet in every home. A heat pump in every home. An induction stove in every kitchen. Phase out fossil fuels and power everything with Pumped Storage Hydropower and Geothermal. I'd make the US go Metric.
But my truly crazy, obsessive idea would be to bring back the French Revolutionary calendar. Or I'd purge all French influences from English.
I'd make all detached single-family homes near an urban center illegal and start replacing them all with a combination of low-rise attached units for families, and apartment complexes for people without kids (notionally, I'm fine with some intermixing). Make sure that there are services and amenities within walking distance of all newly-densified housing, and run bus service and light rail everywhere. Then I'd ban private vehicles anywhere within view of a skyscraper. You will live in a pleasant, walkable community with greenspace and accessible services, or you will be sent to the reeducation camps.
Also, big bronze statue in a park. Big enough to provide ample shade to park goers in the summer.