With the release of OE, Fatshark changed the way active ability worked, from a single instant ability to a special weapon working on a ability charge
Specialist Weapon in Darktide could really well get this treatment.
Plasma/Melta/Flamer/Heavy-Autogun for example would be a Special slot for the Guardsman while his regular weapon would be a Lasgun/Autogun and a ChainSword
While the Ogryn would have as career skill either the Grenade Launcher, or a Melee Charge
The Psyker in this case would work like the future Sienna career (whatever that may be)