Rarity mudwrestling? I might be okay with this!
Prediction and wild guessing time! Also assumptions galore!
Anywho, apart from hoping for possible mudwrestling, I'm going to assume AJ doesn't have an interest in Trenderhoof here. If she does, Rarity is going to look pretty bad, trying to possibly go between others. Yeah, people do that, but these are supposed to be good friends and Rarity if anypony should be willing to share! Generosity, after all!
So yeah, probably AJ will be all awkward because of the situation, or maybe tease Rares about it, but I don't see her taking any advances seriously. Maybe her goading gets Rares to step up her game or some such.
However, Rarity going so far as to try and be somepony else for somepony she hadn't met before seems weird. She is a lady, after all! Of course, this could be an extreme reaction to the Gala incident where she expected everything from the one she wanted and now is willing to do whatever to get them. …that sounds rather desperate. Well, I guess we'll see what actually happens.
If we don't have Spike trying to impress Rares or act aggressive towards Trenderhoof I'm going to be surprised. Oh, and the lesson is probably going to be about being true to yourself, since trying to be somepony else and living a lie doesn't make you as happy, or something similar. Mayhaps she gets Trenderhoof's attention but ends up resenting what she had to do to get it.
I'm a bit wary of them tackling romance in the show but if there's one pony who they should use with it, it's Rarity. Rarity's view on romance and relationships so far seems to have been from a storybook with the prince on a white horse - not like that, you dirty person, you, it's a figure of speech - and I am not certain it meshes well with what's being conveyed here. But all I've seen so far is the synopsis, so this is all just pointless speculation. I'll have to see how they do this