Man of Steel II - Supergirl Begins
If they want to go the replacement route that is IMO the only way to go if they don't want to loose the (DCEU) Supes fans. Maybe they do not even want to replace anymore after that and then have two legs to stand on. Even if they actually do not want to replace at all it can be a great story. And it could totally involve Brainiac and his toy city fetish as a reason for them to meet up to begin with.