Being any kind of designer means looking at the world and seeing it for more than just it's surface features. It means seeing the patterns that lie beneath and reaching conclusions about how those patterns have been shaped by forces such as functionality, intentions and motivations of the concept's originator, the way a viewer or participant engages in the design, and considering where outside forces such as social values, economic constraints, and technological barriers have impacted on the final product and it's experience.
(Sorry, I've spent too long studying design, art, sociology, and para-linguistic communication at university).
Being a TTRPG designer means creating a system that may be used to emulate an experience on the table. Such an experience may strive to emulate elements of reality (but attempting to do so with any sense of comprehensive simulation has always proven problematic in my experience), or it may strive to emulate a specific genre of fiction by incorporating or alluding to certain tropes commonly associated with the genre. Like all design, it means remaining aware of everything else that is going on in the design space, and being willing to accept new ideas, or at least investigate them before jumping to conclusions. It means not being an OSR grognard who can't accept that anything good might come out of the story-game community, conversely it means not being an avant-garde storygamer who only sees the OSR as troglodytic throwbacks. It means constantly refining your work to make it the best it can be, and accepting that at some points your "refinements" are actually just corrupting the design, and that it's time to move onto something new.