I agree with what Silveus says. And if you really want to make a side chapter and don't have time make it related to the MC and the story progression. The most important thing is to have a good story then you can do what you want. Also I feel like what you want is for there to be a separate story in the main story like magical index and railgun.
i upload 3rd chapter of my story today. so guys check it out and give some comment on my writing. today is it better than before or just the same as yesterday or day before yesterday.