Apple may is yet to unveil its first ARM-based iMac, but a YouTuber has. The project is rough around the edges, but it shows what Apple could create if it merged the existing iMac with an M1-based Mac mini.
https://www.notebookcheck.net/DIY-project-brings-the-Apple-M1-SoC-to-an-old-Apple-iMac.526127.0.html