Simple nested prefab utility with editor preview.

PrefabNode.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

#if UNITY_EDITOR
using UnityEditor;

[CustomEditor(typeof(PrefabNode))]
public class PrefabNodeEditor
    : Editor
{
    private GameObject previewPrefab;
    private GameObject previewObject;

    public void OnSceneGUI()
    {
        if (Application.isPlaying)
            return;

        var self = target as PrefabNode;
        if (self.prefab == null)
        {
            if (previewObject != null)
            {
                DestroyImmediate(previewObject);
                previewObject = null;
            }

            return;
        }

        if (previewPrefab != self.prefab)
        {
            if (previewObject != null)
            {
                DestroyImmediate(previewObject);
                previewObject = null;
            }
        }

        if (previewObject == null)
        {
            previewPrefab = self.prefab;
            previewObject = GameObject.Instantiate(previewPrefab);

            previewObject.transform.SetParent(self.transform, false);
            previewObject.transform.localPosition = Vector3.zero;
            previewObject.transform.localRotation = Quaternion.identity;
        }
    }

    public void OnDisable()
    {
        if (previewObject != null)
        {
            previewObject.name = "dead";
            DestroyImmediate(previewObject);
            previewObject = null;
        }
    }
}
#endif

public class PrefabNode
    : MonoBehaviour
{
    public GameObject prefab;
    private GameObject instance;

    public void OnEnable()
    {
        if (prefab == null)
            return;

        if (instance != null)
            return;

        instance = GameObject.Instantiate(prefab);
        instance.transform.SetParent(transform, false);
        instance.transform.localPosition = Vector3.zero;
        instance.transform.localRotation = Quaternion.identity;
    }
}