Blizzard added audio files for other Dracthyr classes in Dragonflight Beta Build 45969. However, this doesn’t mean there are plans to allow more classes besides Evokers, for now at least.
This was discovered by content creator MrGM, who noticed new audio files in the latest Beta build that reference Dracthyr classes outside of Evokers, mainly related to resources (or lack thereof) in the game files. Specifically, “Out of Energy”, “Out of Mana”, “Out of Ammo”, “Out of Rage”.
Blizzard adding the audio files is just for futureproofing. They do not confirm anything at this point and the Dracthyr race will be exclusive to Evokers when Dragonflight launches. Still, it’s good to know they exist in the game files.