Game designer advice: identify your weaknesses and work on them. Are you great at rigorous numerical design but bad at writing compelling, flowing prose? Read more books! Journal every day! Do what your favorite writers do.

Likewise, if you're a writer who can skillfully apply poetic language and literary theming to your prose, make sure you're studying the mechanical systems underlying your favorite games so that your designs are elegant and not hacked together. (This is my biggest challenge.)

You can get by as a game designer by being strong in one of these skills and passable in another, but the best designers strive to be as holistically skilled as they can. And there are tons of other secondary skills that can help you, like clear communication & leadership skills.