So many possibilities...
If Alexandra Moltke hadn't left...Hopefully, they would have resolved the question of Vicki"s parentage. But another question that comes to mind is what would have happened to the character of Maggie. With Vicki still there, she wouldn't have become governess at Collinwood. With both her father and boyfriend gone, what would they have done with the character?
If Joel Crothers, and later, KLS hadn't left, would they have eventually reunited Joe and Maggie? I'd like to think so, as they were my favorite Dark Shadows couple.
If Don Briscoe hadn't left, how would they have resolved the story of Chris' werewolf curse?
And lastly, what if Jonathan Frid hadn't insisted on playing a new character as a condition of his remaining with the show? Would they have ended the show with a story set in regular time, which IMHO would have provided a more satisfactory ending to the series.
A very thought-provoking question, Miles-23!