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.
Hobby centers where all of the gear you would need to get started on various hobbies are available for anyone to use on site. Like a library loaded with tools, CNC machines, stuff for knitting, high end computers, art supplies, musical instruments and recording gear, etc. I would limit the ability to take things off-site only to people who cannot visit in person for medical reasons.
The goal is building community and encouraging creating things for fun. It would also promote innovation, art and music, etc, but that would never be the goal, just a nice side effect.
These are already a thing in some places at a small scale. Libraries in some areas lend tools, and there are some community tool libraries that either lend tools or lend tools and have workspaces!
Mine was pretty hostile to anyone noticeably alt :(