Quoted By:
>the champ gets complacement and far too sure of himself
>an old ally turns their back on the champ out of jealousy
>a new challenger appears out of nowhere, even stronger than the foe our champ won the title from
>a friend or mentor is injured, making the feud personal
>the champ's old rival becomes a new friend
And this is all from a single screenplay. There's plenty of options, if the creative team cared to try.