Basically what the title says; plotlines that never finished (https://tvtropes.org/pmwiki/pmwiki.php/AbortedArc/LiveActionTV) not because the show was cancelled but because an actor (https://tvtropes.org/pmwiki/pmwiki.php/Main/AbsenteeActor) from a show either left (https://tvtropes.org/pmwiki/pmwiki.php/Main/RealLifeWritesThePlot) on their own or was fired.