I totally misread this question at first! I was thinking it asked, "What crazy idea today will be commonplace in the future?" rather than the other way around. I'll answer both.

Common today, crazy in future: using cash that's actually made of paper or metal

Crazy today, common in future: (1) never, ever, ever leaving one's home to work, shop, play, interact, work out, etc. but merely logging into the metaverse and (2) everything, everything will be NFT-ified.