Power levels.
Try not to overpower characters too quickly OP characters can be alot of fun but many stories can be ruined if they go from Issei with no boosts in S1 to Goku SSJSS in a chapter.
My advice is to use A dungeons and dragons players handbook and use that as a guideline levelling up no more than once per chapter although this wont help with everything it should help keep your characters at a decent but not stupid rate.
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guidelines to writing a great fanfiction
Non-Fictionthis whole document is my opinion mostly do as you wish but I hope this helps people out there
