I can see it being Navy Breacher unironically, despite people going literally WHO and asking WHY once it's out
Arbites exists because it let them put focus on underused weapons so far that were Shotguns and Mauls
Navy Breacher would come with focus on Axes while also being soft update for Arbites by giving him Shield and Shotgun which would come with Navy guys. Also easy 2h Axe ports from VT2 and 2h Chainaxe which could be added to other classes
C.A.T. Unit and explosive Gheistskull could make use of npc ally Dog system. In options when switching from double tag to single tag controls, game even talks about generic npcs "ally" and not dog in particular.
Could add meltagun and las-volley as well via paid dlc like this. It would be basically paid update to Vet, Zealot and Arbites and super easy to do, while fitting theme of the game