A popular complaint about the sequel is that it will be less focused on Superman, and it’s basically Justice League.
I think this is a bit off the mark. Let’s all remember that Batman and Wonder Woman are Bruce Wayne and Diana Prince. They aren’t fighting crime in costume all the time. For all we know, they’ll only suit up at the very end and most of their appearances will be as civilians.
It seems more likely that the movie is a precursor to Justice League and is still very much centered around Superman. Before Justice League happens, they have to explain how the idea to create a formal team got sparked in the first place. This movie can explain how the Trinity got together. It can show what lead Superman to meet Diana and Bruce, and why they started to work together. The Trinity has to be established first because they’re the core of the Justice League.
I think the movie will start a few years after the events of MOS and deal with Superman butting heads with Lex Luthor. Let’s remember that Superman lost all the remaining Kryptonians so aside from Lois and Martha, he’s still very much alone. And when Superman needs saving, the Dark Knight appears with an Amazon Princess. Superman finally found new friends that are his peers. Super Friends! xD
Hmm, who else can they recruit at the end? Cue The Flash.