Sorry, didn't get that :)
IMHO root is the only practical default as this will always work out-of-the-box. 

Is there a way to enforce configuration settings in sbt? The idea would be to add a setting `appUser` which has to be set.
This setting is used to set default `daemonUser`, `user` and `group` permissions for all `packageMappings`. The API could
look like this

```scala
appUser := "player"

// This would be the defaults
appGroup <<= appUser

daemonUser <<= appUser

defaultPackageUser <<= appUser

defaultPackageGroup <<= appGroup

useDefaultPackagePermissions := true

```

This would create a lot of new configuration settings and essentially a  bunch of new features. 
However we can enforce the user not using root as default.