Well, there really wasn't going to be any "Laura" as we knew her... The 91 series would have had Lysette Anthony return as "Laura," but she was really more like "Cassandra." It was unification of characters. Like we've recently gone into over Maggie.
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It would have been sad to see a real original character like Laura the Phoenix blended out of existance... that's the direction I think the 91 show would have gone into, had it continued.
Actually, wes, the '91 series writers weren't quite sure how they might handle Laura.
From SHADOWS IN THE '90s: The Dark Shadows Concordance 1991: The writing staff was quite interested in developing Roger's character - the romance with Maggie and the mystery about his wife were elements they intended to explore further. Laura Collins would have been an integral part of the storyline. They discussed many different options. It was suggested that Laura would actually be Angelique all along, but it is doubtful they would have followed that particular avenue. Laura would definitely be identified somehow with Angelique - and it was intended that Lysette Anthony play the role - without actually being Angelique. She could, perhaps, have been possessed by Angelique prior to her institutionalization. Or she could be a phoenix with supernatural powers of her own, as she was in the original series. The 'catfight' of all time was envisioned - a battle of witches. There would also be a nod to the Gene Tierney "Laura".
The book also goes on to say that the writers "did intend to bring Angelique physically into the 20th century, to incarnate her somehow."
So, given all that, it's not actually clear that a union of Cassandra and Laura would have been the way they were definitely going to go. For all we know, the nighttime Laura may have resembled the daytime Laura in many respects - or not at all.