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.
Venus would be more viable
I'm not entirely sure of that, Venus just has so much atmosphere. Most of the "easy" methods would turn it into 4x10^19 kg of graphite, or roughly the mass of Saturn's moon Mimas. Now that's a lot of pencils!
Direct atmospheric removal would need to carry it sufficiently far away from Venusian orbit as to not result in recapture, perhaps exporting some of it to Mars is a decent use?
I think a couple maybe-possible options could be used together.
Drop 4 Vesta into Venusian orbit as a satellite, mining it for primarily Magnesium and Iron, as well as any Calcium present. React with the atmosphere to form carbonate rocks and to leave Iron behind as a catalyst for the Bosch reaction we're about to use to convert the remaining atmosphere. Move Miranda from orbit around Uranus, impacting it into Venus at an angle to add some (not a ton) of velocity to its rotation, and to eject material from both the surface and atmosphere. Most material can be incorporated into Vesta, or atmosphere could be sent to Mars if you want. The primary motivator for impacting Miranda is the large amount of water ice (and hydrogen) which will introduce substantial quantities of water and will strip additional atmosphere through the Bosch reaction mentioned earlier.
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Yeah, could freeze the atmosphere and then ship that to Mars. That's also another solid option instead of using the Vesta Moon proposal I also replied with. It's not about difficulty, it's about fun! Just dropping a big dipole magnet in front of Mars and maybe another big dipole magnet (and i guess a solar shield depending on viability of construction) in front of Venus and another is really all I wanna do right now til we can figure stuff out.