Things that should work in theory...
Lots of big ideas out there, but some just aren't meant to be put into practice, at least not now.
1. Drain the Mediterranean to create more land
See the Atlantropa Project
2. Build a space elevator
Works in theory. The problem is, how do we manufacture a 22,000-mile long cable that won't break?
3. Build a waterway over the Alps
Someone in the early 1900s devised a plan to allow ships to cross the Alps.
There are geeks on Youtube working on it
5. Stop Hurricanes
The US government has tried it in the past.
6. Time travel
I remember seeing a patent application for a "time travel" device. It involved putting cameras and sensors everywhere on Earth and recording every aspect of history on Earth. That way, you can recreate any recorded time on Earth and "travel" back in time, as well as "teleport". Like all fantastical devices that are impractical to build, a patent was never granted for this one.
7. Cold fusion
8. Terraforming Mars
We could do some engineering on a massive scale on Mars to create an artificial atmosphere that Earth life could live in.
Communism is actually a great idea on paper, but doesn't work out so good in practice.
10. Training big dogs so small people can ride on them
If the dog is big enough, and the person is small enough, I don't see why people can't ride dogs like horses.