There’s not currently a way to create new death nights and the death nights we see being created exist in a time before the current date. Also, the new Allied race shares a very similar history with other Allied races who are also not allowed to be death Knights. they were hidden away for thousands of years away from society and were not part of the war in which discourage resurrected the dead of other races. It would be nice at some point if they simply did a race class faction unlocked, allowing everyone to play what they play where they want to play it and enjoying themselves even more. Unfortunately, until then, there are still some stories specific aspects that have to affect which new races get which classes. DK is probably going to be the most likely left out until we bring back a race that existed during the time of the war, such as ogres.