I admit I haven’t heard about these rumours, but if WB are willing to invest $50-$60 million into this cut, then not only are they expecting big things, but they’ve gotta be counting on a future following these specific events, so I’d say yeah. If this cut is a success, you can bet we’ll see a JL sequel within five years; the only big question is what they’re going to do with the Batman.
Not really. Darkseid might be making an appearance in Snyders cut, but (and from what we suspect we know) it’ll only be in the flashback, knightmare and end credit scenes. He’s not going to have any direct league confrontation, and thus remains a baddie for future films.
Brainiac, at least in my opinion, should be saved as a villain for when they’re able to get Superman and Supergirl on the big screen together and focus the story on them; if they take the story off world too, they’d not need to explain the absence of the league either as they’re (primarily?) earthbound.