I disagree. You HAVE to have a Joker-Batman showdown in every single universe for me to take it seriously. They're not going to tease it in Suicide Squad and have these characters in separate films only to pull the plug on it before it even happens. It's what the general audience is waiting for, it's what the majority of the fans are waiting for.
Sure Killer Croc can have a scene, or there might be a secondary threat. But Joker has to have a huge part in the next solo film. They won't let that slip through their fingers, not a chance.