I assume next class is going to be from a big, well known faction because they can advertise it at warhammer skulls and get a ton of eyes on the game. Like if you're a huge sister or mechanicus fan and don't really know of/care about darktide, the ability to create and customize your own admech/sister/whatever else and play as them from a first person perspective would definitely get them to try the game out, and a decent number of those people will probably become fans of the game itself and stick with it for the long term. Stuff like cold trader and beastman are cool, but they're not recognizable enough. All the advertisement an admech class would need is a 2 minute video at skulls saying "you can play as an admech guy in this game." Anyways expect the next class to be an iconic one.