Easy way to ruin anything - introduce a new character who is either the child of or a close relation to the main character (or a central character). They will be the source of everything that is hate for intruding upon the sacred circle of the major cast, or just ruin the show by making it take a left turn on stupid and pointless. See: Wesley Crusher, Dawn Summers, Kyla Davies, Scrappy Doo, Cousin Oliver, Chibi-Usa (aka Mini-Moon), Mutt, Connor etc etc...
Or else, replace a beloved character who was killed off or left the cast with an obvious clone who acts like an absolute tool and lacks all the charm and grace of their predecessor. See: Jason Todd (the Robin the fans voted to die), Dr. Pulaski, some hate has already been felt for Project Zero and Grunt, the musical-chairs treatment of L&O ADAs has earned some of them massive hatred, or just see how the last two seasons of Charlie's Angels ruined the show...
The lesson here is don't kill people off.