May I ask (even though I'm time-traveling through the posts that were made while I was asleep) what your main fandom is? That sounds terrific.

My main fandom is MacGyver, where it's a canon trope to introduce one or more new characters into each adventure, including a love interest. It is really hard to steer clear of the Mary Sue waters, at least superficially (although canon also requires that the Lady Scientist of the Week be smart, capable, fully developed as a character, and not interested in a long-term relationship).