A different ending
I do not own Les Mis or Shoujo Cosette
This is an idea I got from a scene from Shoujo Cosette, but I am going to apply it to the characters in the recent movie. (I believe that Gavroche would be about 5 and Cosette 10 when they first knew each other in Shoujo Cosette, so that would make Gavroche 10 and Cosette 15 at this point)
"Gavroche!" Enjolras shouted after the little boy was gunned down. He seemed to be dead. The boys on the barricade couldn't believe that the soldiers would shoot a child and from the look on some of the soldiers' faces, they couldn't believe it. A dog darted through the soldiers and dragged Gavroche's body through the shadows. The boys could do nothing, but continue to fight.
Cosette was staring out her window, wondering about Marius when she heard familiar barking. She went downstairs and opened the door, to see a familiar dog standing protectively over a small body. She recognized the dog that had been a comfort to her when she was a child being abused by an innkeeper and his wife.
"What is it boy?" She asked. She knelt beside the small body and turned him over, his blonde hair framing his face. Cosette gasped. "Gavroche" She whispered. She had not seen him in five years. Tears filled her eyes. His eyes opened and he saw her. A small smile graced his lips.
"I missed you" He whispered. Cosette lifted the boy in her arms and called for one of the servants.
"Go fetch the doctor quickly" She said.
Later, after the battle.
Marius awoke in a strange bed. His arm was bandaged. As he sat up, he noticed the man who had volunteered at the barricade and Cosette. He put two and two together.
"You're Cosette's father" He said in a soft voice. The man nodded.
They heard a commotion outside the door.
"I want to see Marius" A familiar voice demanded. Marius's heart leapt. It couldn't be, could it? Another voice answered the boy.
"Master Gavroche, you have not fully recovered" A woman pleaded with him.
"I don't care" Gavroche said. Cosette's father opened the door.
"It's alright Marie" He said. "Let him in" Gavroche entered the room. He was shirtless, bandages covering his stomach and every step he took caused him to grimace in pain. He walked over to Marius's bed side.
"I thought you were dead" Marius whispered.
"So did I" He replied, "but Chou Chou, my dog took me to Cosette" Marius looked between Gavroche and Cosette.
"How did he know to bring you to her?" He asked. Cosette was the one who answered, while she forced Gavroche to lie down on Marius's bed.
"We played together when we were children" She replied. "Gavroche took care of me even when his parents were cruel to me" Marius didn't understand. Gavroche looked up at Cosette.
"Eponine is dead" He said softly. Cosette gasped and then gained a determined look on her face.
"Gavroche, you will stay with me" She decided. A grin lit across the boy's face.
"Permanently?" He asked.
"Permanently" She replied.
After Marius and Cosette married, they officially adopted Gavroche, not really as a son, but as a younger brother. Jean Valjean finally told Cosette the truth about his past and offered to leave them, but Marius and Cosette surprised him by telling him no. Gavroche told Valjean that he has stolen food before to feed himself and Eponine and that Valjean is not guilty of anything and has redeemed himself with the kindness he showed Marius and everyone else at the barricade.
The small family lived happily in Paris until Valjean passed away. Then Marius moved his family back to the small village where Gavroche and Cosette had lived when they were younger and rebuilt the Inn. They called it Eponine's Inn and had a wonderful fulfilled life.