When I did my Charles-Delaware-Trollish version of what-happened-if-the-show-continued, I concluded the series (ending it in the late 70's) with a storyline that had not only a banshee, but all sorts of Irish folklore creatures that kill everyone and destroy everything based upon a curse placed upon the Collins family in Ireland in the 1500's (before the family relocates to England, and then to America). Of course, the only ones to survive are Barnabas and Angelique (who had, in my CDTish, reformed several seasons before) and both return to that Irish 16th century past to stop the curse. Imagine the costume extravaganza in that one. I ended it all with a real bang.
Gerard