What’s your favorite concept car that never made it into production?

Cadillac Cien my beloved. Let's take the Northstar V8, already known for head gasket issues, and add 4 more cylinders. What could possibly go wrong??

And I want one so badly 🙁

I'm aware Cadillac finally remedied the head stud issue by 2004, but the Cien came out in 2002.