I've been wondering what approach they could take that was neither a copy of Marvel's (solo hero films followed by mass team-up) nor a simple inversion of it (mass team-up followed by solo films). Maybe what they could do is the equivalent of a team-up comic: Do several Superman films, each of which teams Superman with a different guest hero. So that Batman is introduced in the second Superman film, Wonder Woman in the third, etc., and then it eventually builds to a JL film.
I'm not saying that's what I think they are doing, but it just occurs to me as something that might be suitably distinct from the Marvel approach.
I thought the same thing and it's probably the case.