Given the way that the AllowedTypesAttribute works, you have to specify each type that you might want inside that ContentArea, no inheritance or interfacing allowed. I came up with a fairly simple workaround for situations where you want to control the allowance on all ContentAreas that have no AllowedTypesAttribute defined in the solution. Mainly to avoid having to enter a lot of text.

Step 1: Create an interface

Create a new interface, preferably somewhere where interfaces like to grow.

public interface IContentAreaContent : IContentData { }

Step 2: Create a configuration class

In the same project as your models. Or modify the below example accordingly.

public static class ModelConfiguration
{
    public static readonly Type[] DefaultAllowedContentTypes = Assembly.GetExecutingAssembly()
                                                                        .GetTypes()
                                                                        .Where(p => typeof(IContentAreaContent).IsAssignableFrom(p) && !p.IsInterface)
                                                                        .ToArray();
}

Step 3: Create an EditorDescriptor

This step overrides the ContentAreaEditorDescriptor and specifies all the classes that implement IContentAreaContent for you.

[EditorDescriptorRegistration(TargetType = typeof(ContentArea), EditorDescriptorBehavior = EditorDescriptorBehavior.OverrideDefault)]
public class AllowedTypeContentAreaEditorDescriptor : ContentAreaEditorDescriptor
{
    public AllowedTypeContentAreaEditorDescriptor()
    {
        AllowedTypes = ModelConfiguration.DefaultAllowedContentTypes;
    }
}