Jerome Valeska and Jeremiah Valeska were my top favorites in the show. I found the Valeska Twins to be great interpretations of the Joker, even though One of them wasn't the actual Joker, but I sort of wanted to create my own take on a 3rd candidate of The Joker alongside Jerome and Jeremiah in the Gotham series.
I hope this idea doesn't sound too bad to You:
When Jack Napier was a Young child around the age of 10, he grew up with Awful parents and eventually ran away from home. The Mobsters of Gotham City found Young Jack On The Streets of The city, and then he was raised under The Mob Boss Salvatore Maroni.
Now Jack Napier wouldn't really talk about his past childhood because he wants to move on from his awful past & to keep A bit of mystery to him since He's the Future Joker After all. I wouldn't have Jack be friends or interact with Bruce Wayne a lot since he's A lot different from him in some ways and grew up on the hard streets of Gotham City after running away from his awful family. I'm Not saying Jack would dislike Bruce, but I'd want to make his journey to who he becomes different from Bruce's since they're opposites.
So For The Next 7-8 Years leading to The events of GOTHAM's current timeline, Jack Napier is The Youngest Gangster of Gotham's criminal underworld at the age of 18-19 and helps Salvatore Maroni's Henchmen with Bank robberies to build his criminal empire, Jack would succeed with some Robberies, but he started off rough.
Now, As for how Jack covers his crimes for Sal Maroni to avoid being caught by Jim Gordon and the G.C.P.D, I'll probably say he wears A Clown Mask and Clown-Related stuff to disguise himself from getting caught and foreshadow him becoming The Joker when Batman comes around. Due to Jack Napier's experience with growing up on the tough parts of Gotham's criminal underworld, Jack became better overtime with his criminal career.
For starters, Jack would have A natural aptitude for chemistry & did work in drug labs for Salvatore Maroni for an undisclosed amount of time, So for that part of Jack Napier's character, I added A little bit of A Breaking Bad-type of part in his backstory. You ever watched Breaking Bad before? Trust me it was a great show.
Anyway, Not only would Jack have A natural aptitude for Chemistry which made him an expert in Drug Labs, but he's also An expert in street-smart brawling & knife-fighting since he grew up under Salvatore Maroni & in Gotham's criminal underworld, but also great with shooting and has A nice talent for scheming, Similar to The Dark Knight's Joker, since he was raised in Gotham's criminal underworld for Years after all.
Jack Did succeed at times with his Robberies for Sal but he had A Few Hits and Misses, which led to Sal Maroni beating him at times & leaving him on the streets sometimes until he started A secret Partnership with Oswald Cobblepot & Carmine Falcone who saves Jack's life in a gang war between Carmine Falcone and Sal Maroni.
Jack Napier eventually managed to help them win the Gang War, which led to Oswald Cobbepot & Jack Napier getting higher in the ranks of the criminal underworld. As for Jerome Valeska and his Twin brother Jeremiah, maybe they can still be around and meet Jack Napier at some point in the events of the show.
I'd most likely have him interact and partner up with most of the other Gotham Villains like Carmine Falcone, Sal Maroni, The Penguin, Maybe have tensions with other characters Like Ivy Pepper, Selina Kyle since she's a thief and the Riddler at some point.
So what do you think of my Take of Joker's Origin for GOTHAM?
I kind of took inspiration from Heath Ledger's Joker's Scar stories and Joker's Origin from Batman: Mask of The Phantasm for this Joker origin Idea for GOTHAM.