I doubt it's one of the main characters, I'm hoping its just a ploy to create some hype & they end up some extra that nobody cares about
(((Please don't be Meg)))
My bet's on Chris. I noticed when they mentioned the Simpsons crossover, no apparent plot line for Chris. They paired Stewie up with Bart and Meg with Lisa.
Although, I kinda hope it's Brian. Get a new pet, one that doesn't have the most annoying opinions on everything.