The first point to consider is where the ending begins. "Thank You, Ma'am" is short even by the standards of short stories, so the part that counts as the ending is not a lot of text. Perhaps a good place to start would be immediately after Mrs. Luella Bates Washington Jones tells the boy to eat some more, replacing the last three paragraphs of Hughes's text with your own idea.

The story as it stands is of the heartwarming variety, although there is a certain sadness in the final sentence, telling the reader that the boy never saw Mrs. Jones again. It could be made more heartwarming by changing the story so that the two become good friends afterwards, though, in this instance, you would have to be careful not to extend the story for too long. There is also a danger of your story being too similar to Hughes's original.

To ensure that your story is completely different, you could make it much darker. The story begins with the boy as a criminal and Mrs. Jones as his intended victim, so it would be ironic if Mrs. Jones turned out to be a worse criminal than the boy. Perhaps she has poisoned his food and sits and watches him die at the end of the story. Or perhaps she runs a gang of thieves and sees potential in him or wants to give him better training.

One important point to remember is that the title is related to the boy's words in the final paragraph. To retain this link, you should find a way to make the boy (or someone) say, "Thank you, ma'am."

The point of Langston Hughes' “Thank You, M'am” is that people can create a caring, loving world by their responses to the conflicts they encounter in life. He uses Mrs. Jones' surprising behavior to Roger's attempted thievery to demonstrate this idea. Instead of punishing Roger for trying to steal her purse, she treats him with kindness and even gives him the money he wanted to steal from her in the first place.

However, it is possible that Mrs. Jones could maintain the same attitude toward Roger, but act differently. To use your example, suppose she called the police? Although that is a much different response than what she actually did, it could be framed in a caring way.

Whatever ending you choose to create, you want to make sure that you maintain Mrs. Jones' proper character. She is not going to call the police out of anger or fear. However, she might make the case to Roger that it is in his best interest that he learn right now, before it is too late, that behaving like a criminal is going to make for a very difficult life. She might still cook for him, and perhaps she could offer to help him in some way once his legal situation was straightened out.