My MC has a love interest who turns out to be a younger version of her worst enemy. (The main plot of the book is time travel.)
Two male characters (one is the romantic interest of the protagonist, and the other has been her best friend/rival for years) initially don't get along at all due to their conflicting personalities, but end up getting to know one another slowly over the course of the series and form a very strong friendship by the end. (I think I actually enjoy writing their slow-building friendship more than the MC's romance, whoops)