@ssoapbubble well, you can be original with both (of course, that's depending on what type of fanfic you want to write).
as someone who writes fanfics, they are very fun to write. poetry doesn't take as much time and can be a lot of simpler, however.
i guess it depends on how 'dedicated' you will be. if you want to go simple, i'd suggest poetry. if you want to write something longer and something that you can work on every few days, then i'd suggest fanfics :)