I suppose that it would help to define exactly what we mean by "core gameplay." In Rules of Play: Game Design Fundamentals, Katie Salen Tekinbas and Eric Zimmerman describe the concept in this way [quotes by way of Karl Kapp]:
Every game has a core mechanic. A core mechanic is the essential play activity players perform again and again in a game. Sometimes, the core mechanic of a game is a single action. [...] However, in many games, the core mechanic is a compound activity composed of a suite of actions. In a first-person-shooter game such as Quake, the core mechanic is the set of interrelated actions of moving, aiming, firing, and managing resources such as health, ammo, and armor…Sounds simple, doesn't it? We've already touched on this topic repeatedly: por ejemplo, when we examined the differences in mechanics between RPGs and Action games. We've only looked at this in general terms, though; Salen and Zimmerman are talking about something more specific.
A game’s core mechanic contains the experiential building blocks of player interactivity. It represents the essential moment-to-moment activity of players, something that is repeated over and over throughout a game. During a game, core mechanics create patterns of behavior, which manifest as experience for players. The core mechanic is the essential nugget of game activity, the mechanism through which players make meaningful choices and arrive at a meaningful play experience.
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The notion of a core mechanic is a crucial game design concept, and one frequently taken for granted in the design process. [...] Game designers don’t just create content for players, they create activities for players, patterns of actions enacted by players in the course of game play.Let's look at an example to help illustrate this: the Massively Multiplayer Online RPG. What do players actually do, moment to moment, in an MMO, which distinguishes MMOs from other game genres?