Fixing Despicable Me 4 by moving it's story to second movie instead
His Old boss threatened to kill him if he didn't pay him back and Gru not only sabotages him but probably killed his son until the 4th movie changed it to him being in jail. The first movie shows Gru being pressed by this guy to pay back loans and now he sabotaged him and left his son on the moon possibly dead and screwed him over in billions.
Now it works a lot better Gru would hate being a snitch but would need protection from his old boss especially since his old boss tries to go after the girls to teach Gru what it's like to lose a kid. With this movie we would have more time to explore the girls really and how Gru joined the organization and his relationship with his boss who in his prime say groomed Gru to take his place one day as he wanted someone to keep everything he built alive. Who ignored his red flags as his dad gave up on him or dead and his mom focused more on every new guy she meet but him(this was shown in Gru's origins). He wouldn't get a long with these guys out of jealously so it leaves Gru craving a father figure.
Gru would start questioning his choices at choosing the girls or maybe if he could betray the others and get back in. He could have been handed everything he dreamed of since he was a kid and all the humiliation dirty missions he had to suffer for. Edith could work better than Poppy since it's his daughter who we got to see there bond over the movie not a random girl. We know she has delinquent tendencies maybe in this world she could kleptomania it was a way she survived in the terrible orphanage and but can't let it go. Gru tries to relive his glory days with her at first but instead this later helps Gru realize why he was right to put the criminal life behind him.
Not just for their safety but to be a role model unlike his deadbeat dad and neglective mom which led him being exploited getting over his head and doing things he would regret for the rest of his life. It's implied even in Despicable me 3 Gru used to kill people when he pulls out a gun to shoot Balistar but recalls he doesn't have guns anymore. Agnes and Margo could still be there and not compete for screentime with his wife and the baby and maybe even helping Gru reach this conclusion.
I think Margo got the most focus in the first movie to my knowledge being the one he had to be the mom of her sisters before Gru made her feel like a kid again then again it's been years since I watched it. Despicable me 3 would have the plot of 2 in our world would obviously changes but not too much it would still fit maybe Agnes gets to be in spotlight since Edith and Margo got their turn. Maybe Agnes has a crush on the El Macho's son who is using her to get info on Gru for his dad.
The ending of him saving the world and getting married would be a perfect end to the trilogy. Even the side characters get time to be fleshed out and we learn more about Gru and see his growth leaving perfectly tied up.