There are certain characters that the upcoming DC Cinematic Universe need introduced in Justice League (or earlier in Batman Vs. Superman). These characters are crucial to the future of the franchise. Then there are the characters that everybody wants to see, but have no business in the Universe so soon.
Here are the characters who NEED to be introduced early on in the franchise:
1. Wonder Woman
Yeah, I know the rumors. I know the casting. It is happening. We are finally getting our Wonder Woman. And that is fantastic news. She is literally the final piece DC's "Big 3". For Justice League to work, she has to be in place going into the group film. If you compare them to the Marvel films, Batman and Superman are like the early pieces like Iron Man and Thor. The last piece added (which was Captain America) is what really brings the group together. Although I really would like to see Wonder Woman finally, I hope she doesn't play that large of a role in the Batman/Superman film. A glorified cameo that introduces her is the best bet.
2. Robin (Dick Grayson OR Tim Drake)
Robin is arguably more important to DC's future than Wonder Woman. Think about it. With the introduction of Robin (or Robins), DC can expand in many different directions. We could see a stand-alone Nightwing movie, or even a Teen Titans movie. Although he is not necessary for a Justice League film, he would be extremely valuable to the DC Universe as a whole. I personally would love to see a Nolan Robin film that takes place after the Dark Knight Rises with Damian Wayne as the lead. How great would that be? (Rhetorical question. It would be freaking awesome!)
3. The Flash
I am a big Arrow follower. I love the show. It is all kinds of awesome. I love what they are doing with Barry Allen and the Flash. We do not necessarily need the Barry Allen Flash for the DC Film Universe. Just like the argument with the Robin character, why cant we have more than one? There are many different Flash. You could bring Barry Allen from the DC TV Universe, or you could introduce Wally West. This would also work towards a Teen Titans movie with Kid Flash. One thing is for sure though, The Flash absolutely needs to be in the Justice League film.
4. Green Arrow
DC would be missing hugely by not bringing the Green Arrow from the show Arrow into the DC Cinematic Universe. Many fans already see that as their idea of who Green Arrow is. The show is great. Two years in, and it's one of the hottest show on TV. The only argument against it is that many would like to see Oliver Queen as a rival/friend of Bruce Wayne, and there is a slight age different between Affleck and Amell.
5. Green Lantern
I'm going to come out and say it, Ryan Reynolds was not a horrible Green Lantern. The movie was bad, he wasn't. Ok, now that we have that behind us, comic fans as well as casual film fans have already been introduced to Green Lantern. They know his origin. They know the story. He is a huge part of the DC Universe, especially in the new 52. Hal Jordan or John Stewert? It doesn't matter. I for one prefer Jordan, but will DC want to separate there future films from the Reynolds disaster? Only time will tell.
OK, those are the 5 characters that DC NEEDS to introduce early in their Cinematic Universe. These are the characters that they should stay away from, and why.
I love Shazam. A Shazam movie has been hovering just out of existence for more than a decade. DC might try to push it through. They shouldn't. They should wait. He's too similar to Superman to the casual viewer. It would be superhero overload. Too much of a good thing can be a bad thing. Let the Universe flesh itself out before you bring him in. Maybe 10 years down the line DC might have a Robert Downey Jr. problem and have to replace the Man of Steel? The world may never know. Here is what I propose. Picture it, late in the Justice League movie after our group defeats the villain and the world is saved, a news program shows the effect it has on the world and the emergence of other superpowered individuals. It shows brief glimpses of superheroes doing superhero things (think when Tony Stark is watching the highlights of the Hulk and others). That's where you should first see characters like Shazam, Martian Manhunter, Hawkman, Aquaman, Blue Beetle, etc.
I know, its blasphemy. He's one of the originals. He's cool now. The New 52 fixed him. Don't get me wrong, I'm one of the few people who actually like Aquaman, but I feel DC already has too much on their plates. They don't have the time that Marvel had. They have to tread lightly at first. Bring him in after the Justice League. Make him a "Phase 2" character. Give him the intro he deserves. Don't just throw him into the mix and say "the viewer knows who he is. Give make sure he smashes stuff and swings his trident and talks to sharks." DONT RUIN CHARACTERS. There, I said it. It had to be done.