OCs can be fantastic if done right, but there are key words: Done. Right. If you plan to have an OC, get creative with it. None of that long lost lover/sibling bull****. Give them their own story outside of the canon characters (if needed, you can have them appear and not have a big part.) give them their flaws and quirks, little to no clichéd plotlines, and for ****'s sake, none of that shipping nonsense. The idea of an OC is to draw in the audience with a believable character and situations that can be solved in a familiar world, occasionally intertwining with the canon characters if it's absolutely necessary. I mean, I WISH some people knew how to make a proper OC, but all I see is Mary Sues or overall bad characters and awful writing with some of them. I want to see an OC that pulls me in and actually make me care about them and their story. They are few and far in between sadly.
Yeah, little rant there. Carry on.