In the previous episode of "Dragon Ball Super" titled "Goku Black Rematch! The Appearance of Super Saiyan Rose!!" Zamasu went into the future while Goku and Black Goku continued with their fight. The preview of Episode 57 titled "God with Invulnerable Body - Zamasu's Advent" gives a glimpse into what happens in the next episode.
The preview starts with a battle scene where Son Goku and Black Goku continue their battle in the future. Afterwards, the scene cuts to Future Trunks confronting Zamasu. At the end of the episode, Zamasu can be seen grinning at Future Trunks, which might mean something evil is about to happen.
The preview hints at a major plot-line coming up in the upcoming episode and it seems that Zamasu surely have a plan under his sleeve as despite his confrontation with Future Trunks, he looks relaxed and calm; something which is very unusual for the character. Whatever plan he has in his mind, will be revealed to the fans once the episode will air.
The translation of "Dragon Ball Super" Episode 57 reads: "Hey, this is Goku! Wow! We have to fight Zamasu, who's in cahoots with Black? Better brace ourselves or this could get ugly. Let's go, Trunks! What? Attacks don't have any effect on Zamasu? Next time on 'Dragon Ball Super': 'Advent of the Immortal God, Zamasu'. Don't miss it!"
It seems that even Gowasu doesn't have much knowledge of his tea apprentice's true identity and is not aware that his friend is planning for dominating the humankind in the future.