I think there’s some acceptance in the industry that the terms “casual” and “hardcore” have been co-opted by historical circumstances and no longer match their functional meaning. These days most people array casual and hardcore as two ends of an audience spectrum from large (casual) to small (hardcore). What they’re really talking about is how niche a title is.
This would be merely a semantic debate and beside the point, except that folks in the industry also go on to attribute all kinds of other (arguably more genuine) characteristics to casual and hardcore titles. For example, casual titles may have short play sessions, simple interfaces and fast learning curves. Hardcore titles might have deep, complicated rule systems that encourage extended play and long life cycles. By making the original assertion about audience, however, these characteristics falsely wind up at both ends of a spectrum.
To make something casual is to make it accessible. There’s not much more to it than that. Attention friendly, light on commitment, easy to understand, and so forth. These are product characteristics not audience characteristics.
To make something hardcore is to make it more engaging. More content to consume, more variety, more personal.
The question isn’t whether you’re making a casual or hardcore game but two separate questions: does it enable casual play? does it enable hardcore play? The answer can be yes to both.