What's caused the death of the megaproject in the last 50 years? Even if they weren't realized (and most weren't), all these megaprojects were fascinating exercises in creativity and sheer ambition.
Not to mention climate change. If there is to be a megaproject, it has to be an anti-climate-change one. Otherwise we're engaged in the worldwide megaproject of making earth slightly less habitable.
These days labour is expensive and the stock market needs success every quarter, so products are rushed to the market and everything is leveraged through mass production, outsourcing and automation. Our economies are not able to do megaprojects anymore.
The Soviets were really big into mega-projects (e.g. https://en.wikipedia.org/wiki/Northern_river_reversal, they actual wanted to reverse the Aral Sea shrinking with that too).
Thankfully they came to their senses (and probably also ran out of money anyway).
This for example would create a lot of land but much of it would be pretty salty so wouldn't grow food very well. In addition the remaining water would slowly get saltier and saltier eventually probably dying similar to the Dead Sea, granted this would take a while but it's a definite negative, the Med is already saltier than most of the ocean because it's relatively isolated from the larger circulation of the Atlantic and Indian oceans with a large surface area where evaporation takes place.
Then there are some like the city mega-projects that require a huge amount of social infrastructure that's hard to build, take a look at some of China's mega construction where they've tried to build cities out of whole cloth but the people haven't really come so they're just empty.
This affected a lot of thinking, planning, and creativity. In the 50s-60s people dreamed of megaprojects. Now we look at a megaproject and go: "well, no. it will take too much effort and power with no tangible benefit".
My thought about this was that we ceased the rush for bigger and went looking for smarter.
Atlantropa would have helped there a lot :)
There are the various megaskyscrapers; the top ten tallest buildings in the world are all built in the 2010s, six of them are in China and only one in the West. Of the top 20 tallest buildings, all but one are built in this millennium.
Another project that probably deserves a mention is the ongoing effort to reclaim land from the sea in Singapore.
One Western example is a planned railway tunnel under the Gulf of Finland connecting Helsinki and Tallinn, the capitals of Finland and Estonia respectively. It would be the longest undersea tunnel in the world if ever built.
We're probably going to see quite a few megaprojects related to climate change in the future. Assuming we actually want to stop it and not just talk about it.