I was actually thinking about it with JETX for a time. As little side story chapters added into the Physical books I plan on selling once I finish a story arc since I have everything like that planned out. People said they didnt want little fluff if it didnt add to the story.
I do and still debating on of adding a side story which is more of a continuation of more like, a what if story. Like a bad ending and its just the aftermath of it. However that would be if the story is finished first so I guess that doesn't really count.
Id say do it if you can handle it. But Id also consider if your readers would want to read something like that as well. Mostly you do you, if you think you can. Thats my advice. If not Id say wait lol