Batman

Short answer: Nightwing 

While Marvel keeps introducing new characters to moviegoers, DC seems content at releasing Batman movies, then Superman movies... then more Batman movies, etc. Yes, we have seen one Aquaman, one Green Lantern, and one Shazam movies, and now two Wonder Woman movies.

But mostly, it's Batman's turn, the Superman's, then Batman again, then Superman...

There are many more DC heroes that can be rolled out. Flash is supposed to get a movie soon. But many of the others have not just been relegated to TV, like Green Arrow and Supergirl, they even have to share the show Legends of Tomorrow. 

But the Joker movie gives us the possibility of "a Batman movie without that much Batman in it" (apologies to the Monty Python "spam" sketch). 

Which means the next Batman movie should be a Nightwing movie. 

It really seems that Robin is just a non-starter. He was in, of course, Batman & Robin, which was widely panned. His appearance was teased in The Dark Night Rises... but in the new movie The Batman, he's only teased again. He mostly shows up in goofy, cartoony fare like the Lego Batman movie and the two Teen Titans shows.

Since they clearly don't want us to see Robin in a live-action setting-- and given his outfit, we can see why-- what if we just fast-forwarded to Nightwing?

This character, for those not in the know, is what the first Robin, Dick Grayson, becomes when he grows up. He has learned all Batman can teach him, but he has his own way of fighting crime and graduates from the Batcave. 

Again, DC seems dead set against following the successful Marvel practice of rolling out movies for even its most obscure heroes. They relegate even other Justice Leaguers like Hawkman, the Martian Manhunter, Green Lantern, The Atom, Plastic/Elongated Man, Black Canary, Red Tornado, Zatanna, Firestorm, Blue Beetle, Dr. Fate (yes! that many!) and others to the backgrounds of shows like those in the "Arrowverse" and, earlier, Smallville. 

Batman alone has had nine films in which he is the main hero, starting with the Michael Keaton one in 1989, while-- aside from Superman-- only 4 other Justice Leaguers have had even one or two films each.

Peripheral Batman characters-- like Batgirl, Batwoman, Oracle, and even Alfred, Commissioner Gordon-- have proven compelling in leading roles, and Nightwing did have his own animation. 

Creating a Nightwing movie gives DC what it seems to want-- yet another Batman movie-- and also gives the audience what they want: another DC superhero to root for.