Expanding the DCEU
Nightwing. Black Mask. Deathstroke. Mr. Freeze. Zatanna. Amazo. Those are just a few of the possible heroes and villains that could be seen in this movie. We know that brings up concerns about Batman's solo film being too crowded, but some of these individuals would be nothing more than cameos or have limited roles - and, with a strong script hinting at their stories without feeling the need to blatantly spell them out for everyone, there's no need to make a number of them serve as primary characters. Some of these players are even just there for action scenes, and this could leave moviegoers wondering what the DCEU might have in store for them down the road.
Nightwing and Black Mask could be key players, and they, of course, have a lot of possibilities when it comes to long term plans in the DCEU. Nightwing has spin-off potential and this movie could do a serviceable job revealing what makes him and Jason Todd so very different without needing to fully leap into Grayson's origin story and journey. Because of the tone the DCEU is going for (so far, that is), Black Mask could be a frightening criminal mastermind who focuses on order and the financial success of his territory - the polar opposite of the other villain in the film, Joker. Even if he doesn't get as much screen time as the other primary characters (and he more than likely wouldn't), the source material has quite a few villainous and amusing moments with him that could work well on the big screen while he's also there to advance the plot.
At the time of writing this, there's nothing but rumors about Slade Wilson, a.k.a. Deathstroke, potentially appearing in the DCEU. If he isn't already presented by the time Batman's solo movie opens, this story would be an appropriate way to get Deathstroke fans pumped for the character's role in the universe's future. In the comic, Deathstroke is there as a representative of the Society - a group of villains, and that alone hints at so many other possibilities - and seeing Slade Wilson, in costume, is something many people would love to see in theaters. It would be a great teaser of what's hopefully to come with the mercenary.
Alternatively, WB/DC could surprise audiences by having Deathstroke go after Red Hood (at Black Mask's request) - and once he gets the advantage over Todd, Batman swoops in and then the duo eventually defeat Deathstroke by using their natural teamwork. Needing Batman and Red Hood to work together to overcome Deathstroke would establish Deathstroke as the formidable threat that he truly is. In the comic, Red Hood and Batman's team-up results in Red Hood killing their enemy, so obviously that part would need to be changed so Deathstroke remains alive - unless they want to severely disappoint fans, that is. Deathstroke does have an extensive history with Nightwing, so if audiences react positively to both characters, a Nightwing spin-off featuring Deathstroke wouldn't be entirely far-fetched...
Amazo could be included the same way the powerful foe is in the comic: an android being sent to Black Mask and is intercepted by Batman and Nightwing. This would show that villains are attempting to step up their technology to combat heroes, and the ensuing action scene - which displays Batman and Nightwing's phenomenal teamwork and talent - could be a blast that's packed with spectacle. The writers could even go the extra mile and make fans very happy by including something small like Black Mask saying he's annoyed the deal with Professor Ivo didn't go as planned.
Wonder Woman proves Gods exist. Aquaman proves Atlantis exists. Superman proves aliens exist. But Zatanna? She could tease magic. In the movie, Batman could seek Zatanna out (if he knows who and what Wonder Woman is, knowing of someone powerful like Zatanna is believable) in an attempt to understand how Todd could have potentially been resurrected. Just like Amazo, she could be limited to one scene, but her being involved could tease the existence of so many other characters in the DCEU. Similarly, the Red Hood comic has numerous villains. Mr. Freeze, Hyena, Captain Nazi, and Count Vertigo all encounter Red Hood. If the studio doesn't want to use any of them, there are more than a few fiends at their disposal. Brutale? Cheshire? Rag Doll (who could be very creepy)? Copperhead? KGBeast? The DC universe has dozens and dozens of villains to choose from.
Another option: how about using already established characters? Assuming they survive the events in Suicide Squad, what's stopping Black Mask from hiring someone like Will Smith's Deadshot or Adewale Akinnuoye-Agbaje's Killer Croc? Or Common's character, who is possibly Tattooed Man? At the end of the day, Black Mask simply needs to bring in some extra combatants to go after the very dangerous Red Hood, and there's countless options to use for action scenes and a way to give them a little more depth through their dialogue. (If Deadshot is used, they could pull from the interesting relationship the marksman has with Batman in the pre-New 52 comics.) Choosing to introduce a new and intimidating Mr. Freeze for an early encounter with Red Hood, just like he has in the comic, would also be *ahem* cool. (Sorry.)
An Organic Way to Explore Ben Affleck's Batman's Past
Many would agree that Batman doesn't need another origin story. His parents were gunned down by a criminal, and he vowed to fight crime while dressing as a bat. You all know the story by now, and if you've somehow managed to avoid learning about Batman's origin, a simple line or two of exposition can fix that. Besides, the Batman v Superman: Dawn of Justice San Diego Comic-Con 2015 trailer showed there will be a flashback to Batman's origin.
Yet another origin story isn't something a noticeable amount of fans would want, but a narrative inspired by Under the Red Hood is a great way to look into his past without making it feel forced and it avoids going over Batman's origin for the gazillionth time in a live-action project. No longer is the focus on where Bruce came from - it's now placed on his earlier years with Jason Todd, and how he tried to teach someone who tested his patience more than Dick Grayson did. The Batman being established is older and tougher, but this could show a slightly different take on Affleck's Batman - one who hasn't been destroyed by the loss of an ally just yet. It builds the emotional connection to Todd's journey while it helps us better comprehend Batman's struggle to teach Todd, and then see how drastically the loss changed him. Flashbacks here benefit the overall plot and the development of the two main leads - they're not just for exposition.
The chance to see the Dynamic Duo in action would be a joy for many fans (again, if executed well) and showing Joker brutally attack the young hero would prove this villain is truly a terrifying monster. Given Jared Leto's talent as an actor, we're sure he could make the scene chilling. The BvS trailer hints at Batman's tragic past by quickly showing the Robin costume on display and vandalized by the Joker (or at least we - and everyone else - assume it's the Joker), and the new Batman solo film would have the chance to delve into this dramatic and unforgettable part of the Dark Knight's life.
It'll bypass his origin and go straight to another watershed moment in his life... one which generates a whole lot of action, a solid insight into Batman's methods, and plenty of engaging - and emotional - material. And with Joker already earning the spotlight in David Ayer's Suicide Squad, there's no need to cover establishing the popular rogue in great detail - they can move straight to using him for this story instead of feeling the need to explain who he is and treat his appearance in the new Batman solo movie like it's his big debut.