A person who expiates; someone who speaks or writes at length about a topic.
From expatiate plus the agent suffix '-er' (one who performs an action). This is a rare, literary term.
An expatiater is a lover of words and ideas—the kind of person who, when asked a simple question, will take you on a delightful journey through multiple related topics. Writers and philosophers are natural expatiaters.
Complete word intelligence in one call. Free tier — 50 lookups/day.