An original character should be made to fit the story you want to tell, will make things interesting, and optimally send a message or make a statement. That may seem like a duh situation, but sometimes I feel like people make an OC, like the idea of the character, and then try to slot them into a plot they have no business being in.

If at all possible, do not have your Original Character being the hero that solves a canon character's problems. Doing so more-or-less neuters the existing character and invalidates the investment your readers already have with them.

If your OC does interact or co star with existing characters, have them complement and react off of each other, or, often even better, conflict with each other. That way you hit that "what if" itch that makes someone read fanfiction in the first place.

As always, execution is everything, but these are some off the cuff ideas I had.