This should not be on the most unlikely crossovers pool because of the company connection, Shonen Jump.
Shingeki no Kyojin isn't from Shonen Jump, it's not even from Shueisha.
Nevertheless I still would not say it's unlikely as they're all still connected in how they're all the top of the highest grossing mangas, which would make this crossover no less likely than any Touhou-KanColle crossovers.