Templates are a crutch and to be avoided because they make it progressively more difficult to figure out what MitD would actually be. However, they are not completely forbidden, particularly if you can come up with a compelling reason for it to have the template based on what we've seen in-strip and afaik no one has really come up with one yet.

As GW noted above, if for some reason we get information suggesting MitDs parents are different species, then "half-dragon" or whatever will suddenly become a lot more defensible. IMO, the closest thing we've come across to a justifiable template was the Pseudonatural Barghast Servitor of the Dark One (based on the idea that Pseudonatural means spooky extra-planar and a Servitor of the Dark One would probably be extra planar) and frankly even that's not nearly close enough to being defensible.