To launch from a menu item you would

[MenuItem("Play/PlayMe")]
public static void BuildAssetBundles()
{
    EditorApplication.isPlaying = true;
    EditorApplication.NewScene();
    GameObject go = new GameObject("PlayMode");
    go.AddComponent<PlaySomeMonoBehaviour>().WhatToDo =
        PlaySomeMonoBehaviour.PlayMe;
}

In the PlaySomeMonoBehaviour class

public const string PlayMe = "PlayMe";

public class PlaySomeMonoBehaviour : MonoBehaviour  {

public string WhatToDo = PlayMe;

void Start() {
    switch (WhatToDo)
    {
        case PlayMe:
            StartCoroutine((YetSomeOtherClass.DoSomething()));
            break;
    }
}

YetSomeOtherClass is just another MonoBehaviour manipulating GameObjects.