I wouldn't worry about it. The season isn't anywhere near halfway over, there's still a lot more to see and I'm sure the episodes will start to have some variety.
It would be kinda lame if it was Lightning Dust, reason being because she wasn't really that bad. She just pushed herself way too hard and didn't care about her actions. Trixie was manipulative and did pretty evil things to all of the mane 6 (banning Twilight from Ponyville, making the Mane 5 work for her nonstop, etc). Lightning Dust only had beef with Rainbow Dash and the Mane 5 didn't even know her that well, so I couldn't see her being the focus of this episode. It needs to be a pony who the Mane 6 know well, or at least Twilight knows well. It makes sense that it would be Trixie because Starlight is also very magically talented, so Twilight might fear that she might befriend Trixie and become evil again.
Also, I'm thinking Lightning Dust could be in Newbie Dash, but who knows.
(jesus christ that was long)