The Justice League movie will just be called Justice League
One of the great joys to come out of Batman V Superman: Dawn Of Justice was how easy it was to make jokes about its title. Whatever merits the movie actually has, its title was like a perfect storm of elements that are easy to parody. The “V” instead of “Vs.” makes it sound like a court case, the drama of the subtitle can be diffused by changing a word, and the simplicity of naming the movie after the two dudes who punch each other in it means you can swap in any other two names and it’s clear what you’re referencing.
For example, maybe your friend Tony is mad at your friend Brian for stealing his burrito, so you throw together a photoshop of their faces that says “Tony V Brian: Dawn Of Burrito.” That would be hilarious. Or, going back to the movie itself, you could say something like Batman V Superman: Dawn Of Boredom if you thought the movie was boring, or Batman V Superman: Dawn Of Sequels if you didn’t like how much time it spent on setting up future movies. You could even say Martha V Martha: Dawn Of Martha if—like Batman and Superman—your mom is also named Martha.