Some strange concepts I found whilst browsing are...
The wooden inverted coaster:
And the Fabbri 3-across inverted coaster:
And whilst these two attractions aren't coasters, they features coasters in/on them:
The S.S Disney was Disney's idea of putting an entire park onto a cruise ship. If such a thing existed today, it would probably be one of the most incredible things to ever see!
Here's an article
on the project that never came to fruition as well. I'd kill to see that built!
And finally, an idea of building a theme park... as a skyscraper! Architect Ju-Hyun designed the park to fit into a city where there's little space available for a conventional park:
That actually looks terrifying, and yet, so awesome at the same time!