Turns out that there's an old Equestrian law on the books that any creature can petition to join any princess's harem. Somepony should have warned Twilight, preferably before the former changeling queen showed up on her doorstep.
Harmonic bonds aren't the most predictable of spells. Sunset learned this the hard way with Twilight. Involving Moon Dancer will make things either extremely complicated, or extremely simple. And that has nothing to do with harmonic bonds.
Twilight's just a little nervous. It has nothing to do with Moondancer taking over the Castle Library. Or Moondancer's smile when she suggested taking over the library. Or anything to do with libraries. It's nothing. Nothing at all.
Moondancer has always thought highly of Twilight and was devastated when she left without saying good-bye. When Twilight throws a party for Moondancer and rekindles their friendship, Moondancer decides she wants more.
Moondancer moves to Ponyville after her house is damaged by plunder vines to stay with Lyra Heartstrings, an old school friend. But when she runs into the newly-crowned Princess of Friendship, she finds that her feelings aren't quite what they seem.
Twilight's just gotten back to Ponyville after amending her friendships with her oldest friends. She's happy about that, but still there's something nagging at her.
Moon Dancer was content. Old friends back, getting new ones, having parties, all while still having her studies. But there's still something missing, in the form of the mare who made her realize that there's more to life than dusty old books.
It's taken a lot of time and effort, but Moondancer is finally ready to come out of her shell! Now if only she could find the confidence to ask Twilight Sparkle to come out with her...
Moondancer has spent days pre-planning, planning, arranging, and finally throwing a party for her friends. It would all be for nothing if Twilight didn't show up. Moondancer didn't throw this party just for fun...