There's a lot you could focus on, here. My ideas: Grammar
Vocabulary - books like 100 Words Every Middle Schooler Should Know
Sentence structure
Conversation: Socratic Seminar/Circles
Roots -- Latin, Greek -- medical terms, animal terms, lawful terms etc.
Creative writing (use as a tool to synthesize/identify success regarding all of the above)
Literary concepts that rely on short stories, short films, poetry and/or art.
There's definitely more, but I'll pause here.