Traditionally, a game's core gameplay will determine the genre into which it should be placed, and in turn, this classification will reinforce the genre's definition. The genre itself is usually a nondescript catchall term that is used as shorthand to reference a game's primary features. Broadly speaking, there are many games that share certain characteristics. For instance, there are many games in which the protagonist will possess a health bar. Additionally, more recently produced games will tend to make use of certain industry standards, such as a free reign camera that can be manipulated by a controller's right analog stick, a dedicated lock on function that will allow the player to focus on a single enemy, certain conventional applications toward inventory screens and stat management, and even cutscenes and QTEs. However, and as we all know, some games are not like the others.
The problem lies in how the fundamental terms that comprise a genre are established. For instance, take this list of games:
>Devil May Cry, Zelda: Ocarina of Time, Dark Souls, God of War (2018), Bayonetta, God Hand, Streets of Rage, Ninja Gaiden, Shinobi (PS2), Okami, Resident Evil 4, Nioh
The issue is that games tend to be classified by using a vague amalgamation of the type of game mechanics they feature and their central thematic or literary elements. In my opinion, I think it's more important to focus on a game's primary gameplay experience first, in as specific a manner as possible, and then consider its themes as a secondary layer of the experience. As an example, let's take the term "Hack and slash";
<A game where you use a melee weapon to hack and slash a variety of enemies
This is not enough because, given these conditions, all of the games on our established list will satisfy the classification requirements. Therefore, the term is itself too vague and simplistic to draw distinctions between the aforementioned games. The natural conclusion is that the term itself needs to either be totally destroyed and replaced with better terms or reestablished with more thorough terminology that serves its definition. "Hack and slash";
<A game in which the player can primarily use a melee weapon, or an assortment of melee weapons, and some supplementary weapons, to attack and defeat a variety of enemies, and whose gameplay revolves around a specific character whose moveset has been specifically designed to handle all of the enemies in a unique way.
Under these conditions, Dark Souls (and arguably Nioh) does not satisfy the requirements because it is not designed around a specific character, and the game encourages the player to experiment with different weapons and spells. However, perhaps this still isn't specific enough. "Hack and slash";
<A game in which the player can primarily use a melee weapon, or an assortment of melee weapons, and some supplementary weapons, to attack and defeat a variety of enemies, including bosses that posses a variety of complex moves and phases, and whose gameplay revolves around a specific character whose moveset has been specifically designed to handle all of the enemies in a unique way, and said moveset features a complex system of input combinations that can be combined in numerous ways in order to facilitate a breadth of gameplay possibilities.
Now, the list has been reduced to DMC, Bayonetta, God Hand, Ninja Gaiden, Okami and Nioh. As more stipulations are added, the list becomes narrower and narrower, until a specific gameplay style can be identified. Is it perfect? No, because there is variance between what constitutes qualification in regard to certain elements. For example, Bayonetta is by far the most complex game on the list; how does it compare to Okami and Nioh, whose combat systems are relatively simplistic, despite that they meet the requirements? Still, it's better than the definition we originally had.
All of this then begs the question, "Should elements of gameplay ever be used to define a genre?" As an example, take the horror genre. Horror is generally meant to contain elements that provoke a psychological, visceral, and emotional reaction of fear from the audience. The degree to which such elements are successful does not impact their classification, e.g. whether you think Resident Evil 4 or the film The Shining are scary or not, your feelings on their execution or impact do not change the simple fact that they're part the horror genre because they contain elements that are meant to be scary. In terms of gameplay, Resident Evil 4 is a third person shooter. In this sense, it has more in common with Gears of War or Uncharted than it does with Silent Hill. Yet, both Resident Evil 4 and Silent Hill are meant to be considered horror games.