Is writing a fanfiction a sin?

No, it is not. But be careful of what you write about.

Answer:

If it is the will of God for you to write a fan fiction, then it cannot be a sin, and if it's not, then it cannot be anything but sin "For whatsoever is not of faith is sin." (Romans 14:23) If you fulfill the greatest commandment - "Thou shalt love the Lord thy God with ALL thy heart, and with ALL thy soul, and with ALL thy mind." (Matthew 22:37), then the question would be - "What does God wants me to do right now, and what will please Him most?" and not "Is this thing a sin?", and "Will God let me get away with this?" Jesus said - "If ye love me, keep my commandments." (John 14:15) So first off all - do you love him? Are you so full of his love that your heart overflows with worship and adoration? If not, or you're not sure, don't even bother with with keeping commandments, he doesn't want your obedience if it's not from the heart, he says - "IF you love me"