It seems that while this is technically possible, it would be a very bad idea because a lot of software on Linux assumes the root user is called, well, root.
Theoretically, changing it in
https://unix.stackexchange.com/a/8453 (accessed 2025-03-06)/etc/passwdand/etc/shadowwould be all you need to 'rename' root. The problem occurs because pretty much every single piece of Unix software in existence assumes that the username 'root' exists and that it is the superuser -- mail aliases, various daemons, cron...
It's worth noting that you really shouldn't need to do this, ever. You shouldn't even be issuing commands as root often enough for this to matter. The only reason I have to deal with root is because I'm a developer. I assume the average could get by without ever needing to type sudo in the terminal at all. You can just use the update manager and the software center to install software and their updates.