An unfinished Custom Property Drawer used to select multiple implementations of an interface.

SelectionImplementationProperty.cs

public class SelectImplementationAttribute : PropertyAttribute
{
    public Type FieldType;

    public SelectImplementationAttribute()
    {
    }

    public SelectImplementationAttribute(Type fieldType)
    {
        FieldType = fieldType;
    }
}

[CustomPropertyDrawer(typeof(SelectImplementationAttribute))]
public class SelectImplementationDrawer : PropertyDrawer
{
    private Type[] _implementations;
    private int _implementationTypeIndex = -1;
    private Type type;

    public override bool CanCacheInspectorGUI(SerializedProperty property)
    {
        return true;
    }

    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        //EditorGUI.BeginProperty(position, label, property);

        position.height = EditorGUIUtility.singleLineHeight;


        if (_implementations == null)
        {
            Debug.Log("implementations null");
            _implementationTypeIndex = -1;

            type = (attribute as SelectImplementationAttribute).FieldType ?? fieldInfo.FieldType;
            if (type.HasElementType)
            {
                type = type.GetElementType();
            }
            _implementations = GetImplementations(type).Where(impl => !impl.IsSubclassOf(typeof(UnityEngine.Object))).ToArray();
        }

        string implementationName = property.managedReferenceFullTypename.Split('.').Last();
        for (int i = 0; i < _implementations.Length; i++)
        {
            if (implementationName == _implementations[i].Name)
            {
                _implementationTypeIndex = i;
                break;
            }
        }
        
        EditorGUI.BeginChangeCheck();
        _implementationTypeIndex = EditorGUI.Popup(position, type.FullName + " Implementation", _implementationTypeIndex, _implementations.Select(impl => impl.FullName).ToArray());

        if (EditorGUI.EndChangeCheck())
        {
            object instance = Activator.CreateInstance(_implementations[_implementationTypeIndex]);
            property.managedReferenceValue = instance;
        }

        position.y += EditorGUIUtility.singleLineHeight;

        EditorGUI.PropertyField(position, property, true);

        //EditorGUI.EndProperty();
    }

    public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
    {
        return EditorGUI.GetPropertyHeight(property) + EditorGUIUtility.singleLineHeight;
    }

    public static Type[] GetImplementations(Type interfaceType)
    {
        var types = AppDomain.CurrentDomain.GetAssemblies().SelectMany(assembly => assembly.GetTypes());
        return types.Where(p => interfaceType.IsAssignableFrom(p) && !p.IsAbstract && !p.IsEnum && !p.IsGenericTypeDefinition && !p.IsInterface && !p.IsPrimitive && !p.IsSpecialName).ToArray();
    }
}