Typical fanfics are often riffed with Yang suddenly losing interest in Lina, mostly due to the author Flanderizing her personality into a abusive and selfish girlfriend, or just making her not exist at all so that the author's Relationship Sue can be with him; normally, Yang acts entirely out of character or the OC in question is an overly perfect match for him.
Another very common scenario is Yuck going through a Heel–Face Turn, saving Yin from some sort of villain or trap, and the two of them being together. Expect Coop to be unmentioned throughout.
Ones entirely centered on Yuck are often about him having severe depression or wanting desperately to change and be loved by someone; after "Upstanding Yuck", him fighting against a split personality was very common.
There's also having at least one of the twins become evil, normally Yin after the events of "Gone A Fowl".
Don't forget "Yin Yang Who?", though; with it, you have two things: either