Hey, everyone. I got that idea while reading in Fanfiction and a final scene of Gotham 4x21 episode.

Hopefully, enjoy.

Synopsis: The final scene of Gotham 4x21 shows Jeremiah Valeska/Joker enters the room where Bruce Wayne & Selina Kyle are but before the Joker shoots Selina, someone interrupted and the Joker didn't expect to be BULLIED!
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~At Wayne Manor~

Bruce's eyes shot open and they started darting around the room, searching for the cause of the sound that'd awoken him.

Apparently, his sudden movement disturbed the girl sleeping next to him.

Selina: Bruce? What's wrong?

Bruce didn't respond immediately, still feeling that there was something off. Maybe it was just the fear toxin he'd been exposed to earlier that day, or maybe it was just a bad dream. He wasn't sure.

Finally, seeing no immediate threat in the room, Bruce responded,

Bruce: Nothing. Bad dream I guess.

Selina: Jeremiah?

She asked, pulling her legs up underneath her so they were both sitting up in his bed. Bruce nodded in response.

Selina: *sigh* Why do you think he's so obsessed with you?

He shook his head.

Bruce: Jeremiah said that all it takes is one bad day to drive a person insane. I wonder if my parents dying made me a little insane. Maybe he sensed that. Maybe he wanted to bring it out.

Selina: Well you proved him wrong.

???: To be fair,..

Another voice announced from the shadows. Selina whirled around to find a gun lodged in her chest. Bruce froze, not wanting to set off their attacker. Jeremiah Valeska aka The Joker appears to them...with a gun!

Joker: *crazy smile* The night's not over yet.

~Cue: Drive that Funky Soul~

https://www.youtube.com/watch?v=0oxt-WtC6Oc

Suddenly, a music out of nowhere was played around which Bruce, Selina and the Joker were confuse.

Joker: What the?

The door opened as Bully Maguire appeared, adjusting his coat, then clapping and spinning his hands in a merry-go-round motion before dancing. The 3 were looked froze in confusion.

Bully Maguire: *to Joker* I'm gonna put some dirt in your eye!

The Joker now sneered and took out his gun to fire when Bully Maguire kicked the ground, sending dirt flying into Jeremiah's eyes as he screamed, the dirt blinding him.

Bully Maguire: *taunting* Look at little pathetic rip-off clown! Gonna cry?
Joker: You bastard!!!

The Joker blindly fired one shot, which Bully Maguire dodged, then spinning, grabbed the bullet with a web and hurled it back at the Joker.

It went through his head and he fell down, dead.

Bully Maguire: See ya chump!

After he said it, he then did a dancing spin and walked off to other door.

~End Music~

Bruce & Selina still too stunned to respond just as Bruce's butler, Alfred Pennyworth enter the room.

Alfred: Master Bruce, Miss Kyle! What happened? I heard a shot.

Alfred look at Jeremiah's lifeless corpse in shock & confuse.

Alfred: *To Bruce* What's going on?

Bruce: I have no idea. Call Detective Gordon!

Alfred: Right away, Master Bruce. *leaves*

Selina: Who was that man, Bruce?

Bruce: I don't know but at least, he saved us.

Thanks to Bully Maguire, both Bruce's & Selina's fateful night was change. The Battle of Gotham still happened but the combined heroes & villains defeated Eduardo/Bane & his men. And Bruce NEVER leaves Selina in the aftermath of the battle and stay there for awhile while preparing his journey to the path of a hero/vigilante.

Several years later with the support of Selina & Alfred at his side, Bruce Wayne introduce every criminal organizations and the city of Gotham as the Dark Knight, Batman.
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AND DONE! Bully Maguire beats the annoyingly version of the Joker of Gotham TV series and saved both Bruce & Selina's fates.

What do you think? Please Comment. Thank you.