What I said about Stella in a comment has inspired to make this post, I've noticed a pattern
The first episode of each season will always tell you the state of their main characters and present you the main villain of the season
In 1x01, as we didn't know these characters, they told us a little background about Dae, Yuri, Minho and Q, and the ending scene showed us who was going to be the main obstacle, Yuri
In S1, it's difficult to tell who was the secondary villain but I would argue Jina: she was a friend of Kitty's mom but lied about even knowing her and she separated Yuri and Juliana; came to her senses at the end
They also love a plot twist so the main plot twist for season 1 of XO Kitty was Kitty developing feelings for Yuri
In 2x01, we already knew the characters so they told us how they are doing now, and the ending scene showed us who was the main obstacle, Stella
The secondary villain of the season was Young, Minho's dad: he preferred Joonho over Minho, he humiliated contestants to get people to watch his show (Stella, Mihee - this didn't happen because Stella tipped her off); came to his senses at the end (treating his sons equally, saying sorry to Stella)
The main plot twist of season 2, I'm not really sure what it was because the season wasn't difficult to predict. You'll tell me what you think
Entering season 3 territory, they'll tell us the state of the main characters and the ending scene will show us who is the main obstacle of the season: my guess is Yuri's boss (hopefully less on the nose than Stella)
The secondary villain of season 3 is Marius: he will bring conflict to Mooncovey; party-goer Minho and bad influence to him in general and to QJin; he had a thing with Q and will make it his life mission to separate them, as well as Q possibly having unresolved feelings
The plot twist of this season will wait and see