Board Thread:The Panopticon/@comment-1293767-20151029072618/@comment-27501528-20160110141624

Off the top of my head, instances where these sort of 'cliffhanger' character introductions appear:
 * The 10th Doctor in The Parting of the Ways (if we're counting post-regeneration scenes as 'new guest star' introductions)
 * Donna Noble in Doomsday
 * Simms' Master in Utopia
 * The Ood in The Waters of Mars (if that counts)
 * The 11th Doctor in The End of Time
 * Churchill in The Beast Below
 * Kovarian in The Almost People and Closing Time
 * The War Doctor in The Name of the Doctor
 * The 12th Doctor in The Time of the Doctor

To clarify, this is characters who are introduced into an episode towards the final moments for the purposes of a cliffhanger, who play a prominent role in the following episode. I've probably missed a few.