Trixie had an absurd amount of wasted potential and it still greatly disappoints me to this day. They sat on her character for *years* before finally doing something with her. And I hate to sound spoiled, but even then it wasn't enough.
Even after her return in S6 she was used rarely. You'd think they'd better utilize a character that's been around since the 2009 show bible, but she doesn't even appear in half the amount of episodes Starlight does, it felt like a giant slap in the face to Trixie fans.
They could've dived deeper into her insecurities that they barely touched upon in No Second Prances about her desperately wanting friends to the point of her nearly offing herself because she thought she lost her first and only friend. But they barely expanded upon her character after this, just turning her into Starlight's sidekick and only appearing when she does.
And not to mention the purple elephant (or rather giraffe) in the room, I hate that Trixie and Twilight never really made up, they had them "make amends" *twice*, once in Magic Duel, and the second time in NSP, both of which were pointless because right after this episode they're back to being petty grudge holding brats, see the beginning of To Change A Changeling.
As someone who always wanted Trixie and the Mane 6 to become friends, this really irked me personally. This is probably going to sound like jealousy, but seeing Stariight get everything that I wish Trixie would've gotten while she was instead thrown in the dirt and basically left on the backburner almost felt like it was done out of spite.