One Piece Fanfiction Plot Question?

I'm making a One Piece fan fiction on fan fiction.net and I'm writing about two original characters I made up one boy and one girl. They both are creating a crew together (Pirate Crew or Bounty Hunter Crew) and I just want to know you guys think I should make them both devil fruit users, or do it different and have them just stronger then normal humans like of the thing. I just feel as if the Devil fruit is just over used in One Piece and that every one with them are almost unbeatable to humans.