What's a gigaproject you dare not dream of, but perhaps an ecologically wealthy and healthy planet would start commissioning feasibility studies for?

Sinking England

Give England to Italy. Drive everyone in England at gunpoint to live in a walled-off Westmorland to make room for the Italians who will move in to the rest of the country. Make it illegal for the so-called English to do anything but subsistence farming, and maybe some backbreaking manual labour outside of Westmorland for pitiful wages (and under constant watchful guard). And even then, make sure to murder a bunch of the English from time to time to remind them who's in charge and that they shouldn't get uppity towards their superiors.

And if any English complain, remind them that the Romans were living in and ruling England 2000 years ago, which obviously gives the modern Italians the right to inflict extreme disproportionate violence on any English who dare to complain about this arrangement.
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i read a webcomic where that happens

unfortunately, they just put the cities in bathyspheres

Wholesale biome restoration including returning extinct megafauna. Yeah, some of those extinctions happened because they weren't doing well with the transition the interglacial period, but we have no idea how much adaptation was constrained by hunting pressure from humans. Move the people to more compact settlements and let the ground sloths roam the plains.

You stole my answer
The absence of ground sloths is one of the worst things about the holocene. No offence to two and three toe'd sloths but, like...

I feel like space and terraforming have gotten a bad wrap since their primary purpose seems to be to provide a false solution to climate change.

But, if we did manage to "win" at existing on Earth, we could still get completely wrecked by asteroids, super-volcanos, etc. So some sort of non-terrestrial proliferation of DNA-based life seems like a solid investment. So:

Short-term: space elevator
Long-term: warp drive or some way to bypass speed of light limitations

Or at the very least, give up our anthropocentric biases and rig up a mass Tardigrade or Fungal Spore Yeeter to do our part for Panspermia

Eon-term: gently nudging the sun's orbit through the galactic disc so that in the unthinkably distant future we can make our liquid water zone do a kissy with a red dwarf's liquid water zone, handing off the earth to something with a real lifespan.

edit: we probably want to take Jupiter with us if it hasn't already got a gas giant to herd away the asteroids.

Installing some mountains and water bodies in the American midwest to make the weather chill.

honestly seems like a good place for some mild subterranean architecture. See what it's like, living like a Tolkien dwarf; a few hobbit houses built into artificial hills. Just make sure you have solid water runoff instalations
- Folks in this region did build houses that were subterranean or semi-subterranean back in the day. Mandan, Hidatsa and Arikara people still maintain a couple as cultural outposts.