Yes. Let me clarify. I would make each subclass inspired by the chosen favoured enemy but not necessarily have powers that are only active against them like in 3rd edition.
For example take this fey ranger. They get advantage vs charm and proficiency in a social skill along with some other mind affecting powers. These powers are useful against fey but not restricted to fey, they can be used in other scenarios. The bonus spell list further solidifies this: banishment and dispel magic for dealing with fey, charm person and mislead as fey-like inherited powers.
For a dragon slaying ranger I would give them powers and bonus spells that grant resistance to energy damage types, bonuses to area of effect saving throws, maybe a feature to counter fear. Maybe add in a feature that gives bonuses to attacks when made a certain distance away from your target. These are things that are undeniably useful against dragons, but also generally useful in fighting other things as well.