I don't think I've ever been in a situation where an anatgonist was not presumed to be a staff pet who was going to win no matter what. Often, they'll cite the hero's journey cliche and say something to the effect, "Oh, don't worry, you good guys always lose in the first few chapters, it makes your future never happening triumph even sweeter.
I'm a bit sour to PC antagonists, but thinking about how to do them right, I think having a flag or bboard requests section on players that are LOOKING for an active antagonist would help. This way both players are into it.
Then, as a fan of board games, I'd likely have something like Antagonism Tokens. For each story one side 'wins', they get a token, with consequences getting higher the more tokens have been earned. If one player begins outstripping the other by a far margin, then staff can introduce an outside factor to begin to mitigate things to end the antagonism, like the colony surgeon installing MAD cortex bombs so they play nice with each other.