if there is option of the Administration, do define some of the roles to have the permission to define the Value/DefaultValue of some of the Redmine Custom Fields, and different default value for different project, it would be great.

We can use it to manage the pre-defined values of custom fields directly in the project by role, without turn to Administrator to update the pre-defined value again and again.
And we can set the default value for different project.
Or maybe even, we can set the pre-defined value for different project all or project, different default value for different project or all project.