In the 19th century they have run smack into the Fosse Dangaered, which would have ended the whole project there and them. It wasn't the tunneling technology which would have let them down, it was the marine survey technology, which was really primitive back then.
How about some NZ projects? (noting that we probably couldn't afford them).
One that probably should have happened was extension of the train network into the CBD/south eastern suburbs, via a third tunnel through Mt Victoria.
Silly and useless vanity projects:
1. Bridge across the mouth of Wellington Harbour
2. Bridge across the Otago Harbour from Pt Chalmers to Portobello (god knows why!). It would look pretty spectacular?