PartialSceneObject.cs

using UnityEngine;
using UnityEditor;

/// <summary>
/// このコンポーネントはMainシーンから加算ロードされた場合、非アクティブになる
/// </summary>
public class PartialSceneObject : MonoBehaviour
{
    // 単体シーンのロード時に SceneLoader.cs から呼ばれる
    public void Init()
    {
    }
}

#if UNITY_EDITOR
[CustomEditor(typeof(PartialSceneObject))]
[CanEditMultipleObjects]
public class PartialSceneObjectEditor : Editor
{
    public override void OnInspectorGUI()
    {
        base.OnInspectorGUI();
        EditorGUILayout.HelpBox("このオブジェクトはMainシーンから加算ロードされた場合、非アクティブになる", MessageType.Info);
    }
}
#endif

SceneLoader.cs

using System.Linq;
using UnityEditor;
using UnityEditor.SceneManagement;
using UnityEngine;
using UnityEngine.SceneManagement;

/// <summary>
/// シーンローダー
/// </summary>
[InitializeOnLoad]
public static class SceneLeader
{
    static string currentMainScene = "";

    static SceneLeader()
    {
        EditorApplication.update += disablePSObject;
        EditorSceneManager.sceneOpened += (scene, mode) =>
        {
            if (mode == OpenSceneMode.Single)
            {
                // 追加で必要なシーンをロード
                // switch (scene.name)
                // {
                //     case "Autumn": addScenes("_Lava"); break;
                // }

                // Main以外の単体シーンだった場合
                if (scene.name != currentMainScene)
                {
                    enablePartialSceneObject();
                }
            }
        };
    }

    #region PUBLIC_METHODS
    [MenuItem("Scene/Disable Partial Objects", false, 0)]
    public static void DisablePartialSceneObjects() { disablePartialSceneObjects(); }

    [MenuItem("Scene/Main Autumn", false, 20)]
    public static void OpenScene1() { loadScene("Autumn"); }

    [MenuItem("Scene/Lava", false, 32)]
    public static void OpenScene2() { loadScene("_Lava"); }

    [MenuItem("Scene/Fall", false, 33)]
    public static void OpenScene3() { loadScene("_Fall"); }

    [MenuItem("Scene/Flow", false, 34)]
    public static void OpenScene4() { loadScene("_Flow"); }
    #endregion

    #region PRIVATE_METHODS
    static void loadScene(string scene)
    {
        currentMainScene = (scene[0] != '_') ? scene : "";
        EditorSceneManager.OpenScene("Assets/_Project/" + scene + ".unity");
    }

    static void addScenes(params string[] scenes)
    {
        for (int i = 0; i < scenes.Length; i++)
        {
            EditorSceneManager.OpenScene("Assets/_Project/" + scenes[i] + ".unity", OpenSceneMode.Additive);
        }
        disablePartialSceneObject(scenes);
    }

    static void enablePartialSceneObject()
    {
        GameObject[] gameObjectArray = SceneManager.GetActiveScene().GetRootGameObjects();
        foreach (GameObject go in gameObjectArray)
        {
            if (go.GetComponent<PartialSceneObject>() != null)
            {
                go.GetComponent<PartialSceneObject>().Init();
                go.SetActive(true);
            }
        }
    }

    static void disablePartialSceneObject(string[] scenes)
    {
        foreach (PartialSceneObject partial in UnityEngine.Object.FindObjectsOfType(typeof(PartialSceneObject)))
        {
            if (scenes.Contains(partial.gameObject.scene.name))
            {
                partial.gameObject.SetActive(false);
            }
        }
    }

    static void disablePartialSceneObjects()
    {
        for (int i = 0; i < EditorSceneManager.sceneCount; i++)
        {
            Scene scene = EditorSceneManager.GetSceneAt(i);
            foreach (var go in scene.GetRootGameObjects())
            {
                if (go.GetComponent<PartialSceneObject>() != null)
                {
                    go.SetActive(false);
                }
            }
        }
    }

    private static void disablePSObject()
    {
        for (int i = 0; i < EditorSceneManager.sceneCount; i++)
        {
            if (EditorSceneManager.GetSceneAt(i).name == currentMainScene)
            {
                disablePartialSceneObjects();
                EditorApplication.update -= disablePSObject;
                Debug.Log("Disabled Partial Objects");
            }
        }
    }
    #endregion
}