Instead of a base class keyword with a support for only a name, added an inherit keyword, which just adds an inherit arrow below the current class.

project/BaseClassArrow.tscn

@@ -0,0 +1,36 @@
+[gd_scene load_steps=3 format=2]
+
+[ext_resource path="res://inherits.png" type="Texture" id=1]
+[ext_resource path="res://line.png" type="Texture" id=2]
+
+[node name="BaseClassArrow" type="VBoxContainer"]
+margin_right = 51.0
+margin_bottom = 62.0
+custom_constants/separation = 0
+alignment = 1
+
+[node name="HBoxContainer" type="HBoxContainer" parent="."]
+margin_top = 18.0
+margin_right = 51.0
+margin_bottom = 44.0
+alignment = 1
+
+[node name="VBoxContainer" type="VBoxContainer" parent="HBoxContainer"]
+margin_left = 16.0
+margin_right = 35.0
+margin_bottom = 26.0
+custom_constants/separation = 0
+alignment = 1
+
+[node name="TextureRect" type="TextureRect" parent="HBoxContainer/VBoxContainer"]
+margin_right = 19.0
+margin_bottom = 16.0
+texture = ExtResource( 1 )
+
+[node name="TextureRect2" type="TextureRect" parent="HBoxContainer/VBoxContainer"]
+margin_top = 16.0
+margin_right = 19.0
+margin_bottom = 26.0
+rect_min_size = Vector2( 0, 10 )
+texture = ExtResource( 2 )
+expand = true

project/BaseClassControl.gd

@@ -1,4 +0,0 @@
-extends VBoxContainer
-
-func set_base_class_name(text):
-	$Container/PanelContainer/Label.text = text

project/BaseClassControl.tscn

@@ -1,54 +0,0 @@
-[gd_scene load_steps=4 format=2]
-
-[ext_resource path="res://BaseClassControl.gd" type="Script" id=1]
-[ext_resource path="res://inherits.png" type="Texture" id=2]
-[ext_resource path="res://line.png" type="Texture" id=3]
-
-[node name="BaseClassControl" type="VBoxContainer"]
-margin_right = 51.0
-margin_bottom = 62.0
-custom_constants/separation = 0
-script = ExtResource( 1 )
-
-[node name="Container" type="CenterContainer" parent="."]
-margin_right = 57.0
-margin_bottom = 28.0
-
-[node name="PanelContainer" type="PanelContainer" parent="Container"]
-margin_right = 57.0
-margin_bottom = 28.0
-
-[node name="Label" type="Label" parent="Container/PanelContainer"]
-margin_left = 7.0
-margin_top = 7.0
-margin_right = 50.0
-margin_bottom = 21.0
-text = "Object"
-align = 1
-valign = 1
-
-[node name="HBoxContainer" type="HBoxContainer" parent="."]
-margin_top = 28.0
-margin_right = 57.0
-margin_bottom = 54.0
-alignment = 1
-
-[node name="VBoxContainer" type="VBoxContainer" parent="HBoxContainer"]
-margin_left = 19.0
-margin_right = 38.0
-margin_bottom = 26.0
-custom_constants/separation = 0
-alignment = 1
-
-[node name="TextureRect" type="TextureRect" parent="HBoxContainer/VBoxContainer"]
-margin_right = 19.0
-margin_bottom = 16.0
-texture = ExtResource( 2 )
-
-[node name="TextureRect2" type="TextureRect" parent="HBoxContainer/VBoxContainer"]
-margin_top = 16.0
-margin_right = 19.0
-margin_bottom = 26.0
-rect_min_size = Vector2( 0, 10 )
-texture = ExtResource( 3 )
-expand = true

project/UMLGenerator.gd

@@ -1,6 +1,6 @@
 extends Control
 
-var BaseClassControl = preload("res://BaseClassControl.tscn")
+var BaseClassArrow = preload("res://BaseClassArrow.tscn")
 var ClassControl = preload("res://ClassControl.tscn")
 
 export(NodePath) var content_container_path : NodePath
@@ -105,12 +105,8 @@ func _process_state_next(delta):
 			_content_container.add_child(current_content_container)
 			continue
 			
-		if l.begins_with("base_class "):
+		if l.begins_with("inherit"):
-			var base_class : String = l.trim_prefix("base_class ")
+			var bcc = BaseClassArrow.instance()
-			
-			var bcc = BaseClassControl.instance()
-			bcc.set_base_class_name(base_class)
-			
 			current_content_container.add_child(bcc)
 			continue
 			

project/data/directory.umlg

@@ -1,9 +1,8 @@
 
-class ASd
+class Object
-current_content_container.set("custom_constants/separation", 0)
+inherit
-new_column
+class Reference
-base_class Object
+inherit
-base_class Reference
 class Directory
 
 public: