Well more than likely when we play we will unlock cosmetics that will allow our characters to become "Ard but as an elite class in itself I dont think its going to happen. (As shown in the space marine talent tree they can become Stern Guards)
In terms of playing as them, 'ard boyz are probably along the lines of a LSM Assault & CSM Raptor without a jump pack. Take a stormboy assault class archetype, remove the jump pack to free up load out points for more damage mitigation stats and a shield. That's going to be my preferred style. Well when I'm not firing as many bullets as I can at enemies as a shoota or loota that is. Note that class names could change depending on equipped items as said here.
Thr wot im 'opin for And thats it modt likely whats going to happen in that a 'ard boy will be a grounded stormboy.