I think that all of the cool big bad guys in the MM such as the mummy lord, lich, vampire, etc. should be modified to suit the campaign. These are the types of creatures that I can't envision there being enough of in a given world to be generic. The stat blocks given in the MM serve as a great template or base for such creatures but a DM needs to develop them a bit .
One of the campaigns I am running now features a pair of elder vampires as the main villains. I used the vampire template in the MM as base and built them from there. These elders have powers and abilities the "standard" MM vampire doesn't and the spawn that they create are also more powerful. In addition, a ritual is required to permanently destroy them, else they will simply re-form and exact revenge. The ritual needed is in a book that I added to the treasure trove of Iggwilv converting the Lost Caverns of Tsojcanth to 5E. The final battle with these villains, should the party survive the caverns to fight it, will be quite memorable.