By the way, I plan to make that an epilogue to this story.
Literal
By-the-way, that [topic-は] this story [possessive-の] epilogue make plan is.
ちなみに (by the way/incidentally) is a common discourse connector for tangentially related information. 後日談 means an epilogue or follow-up story — literally 'later-day-tale.' ~にする予定です chains the intention marker 予定 with にする (make into).