NPCs have gotten some attention in recent updates in the form of level cap increases. However, it would be nice to allow them to be accessed in more areas. Some zones that come to mind include Tu'Lia and many zones in the past (ie: Beaucedine (S) and Xarcabard (S)). At the very least, I think any zone that has Grounds/Fields of Valor tomes should allow you to call your NPC.