If a Fighter is hidden and chances upon an opponent, and wants to backstab it, how is it handled, despite him not having a Backstab move?
Or a Fighter who surprises an opponent, and wants to go for a Called Shot, which only the Ranger has? Should I follow what the narrative suggests, or just allow the Fighter to make a Hack & Slash move, or...?
Players do not announce the move they do. They say what their character is doing. Is the fighter trying to sneak up on someone? He's trying to avoid some danger by being stealthy - I'd roll a Dexterity-based defy danger, but of course this changes from table to table.
He sure does not get the mechanical advantages the rogue has, because the fighter has no Backstab move.
He tries to aim at some part of the enemy's body? Offer him a choice. He either hits where he wants, but needs to aim and the opponent is not surprised, or he hacks without thinking but does not lose the element of surprise.
He is not a ranger, so he does not get the ranger's mechanical advantages.
