Robin's origin:
If Matt Reeves doesn’t want to use the traditional Dick Grayson origin, him and his family in the circus, in favor of a more grounded one, I got the perfect one. At the end of The Batman, the city floods. What if Dick’s parents were Gym teachers who taught acrobatics. His parents die in the flood and becomes an orphan. After being inspired by Batman, he dons a prototype suit based on his mom’s gymnastics costume and becomes Robin. After the events of the film, Batman makes him a new armored suit. Batman recommends he ditch the colors. Dick explains the reason for them and Batman keeps them.
Joker is the Bat-Verse Nick Fury:
Crazy theory I hope comes true: Joker is the Nick Fury of the Bat-Verse.
Hear me out.
At the end of The Batman, The Riddler becomes friends with the Joker. But this Joker has already lost to Batman once. Riddler almost beat Batman. Joker knows this, and he could be his first recruit to Joker's "Team".
We know that there are a few villain-based movies in the works set in the Bat-Verse. What if every film ends with that villain losing to Batman and being sent to Arkham. The same goes with however the villain of The Batman Part 2 is.
Once we get to The Batman Part 3, Joker will have his team ready to escape Arkham and kill Batman. But at this point, Batman has a team of his own, Him, Catwoman, Robin, and maybe someone else like Batgirl. This would make The Batman Part 3 a full-on Gotham Civil War!
How characters can get their comic-accurate costumes:
Catwoman- After using Batman's contacts in the film, Catwoman realizes how useful they are. However, she doesn't have the resources that Batman does. this leads her to make a makeshift version with goggles, giving her her classic red goggles.
Penguin- After becoming head of Gotham's underworld, Oz decides to make his classic gun umbrellas to sneak guns into places.
Riddler- In the film, Riddler wears a mask to hide his identity. Now that it's out in the open, if he escapes Arkham, he could adapt his classic look. This could be inspired by Joker's over-the-top schemes.
Note: Did anyone notice that Selena’s mom died the same year the utter trash that was the Catwoman movie came out?