When you've thought of an interesting idea, you can introduce your idea with the expression "You know what would be ___?"

You know what would be fun? What if we all got together this winter and went skiing?

You know what would be horrible? Imagine what would happen if we lost the original files.

You know what would actually be the best way to get there is if you took 540 and got off on the Highway 50 exit.

You say "You know what would be ___?" when you think that your idea is interesting, and you want other people to become interested in the idea with you.