I think a cat-creature would have been cool. I'm thinking of something in between the original Cat People story, the novel The Green Eyes of Bast by fab cult novelist Sax Rohmer (he also invented Fu Manchu and Sumuru), and the 1940s Cat People film. There could have been a brother and sister who were both were-cats as in the Nastassja Kinski version of the story.
I'm not sure that a mummy story would have worked. Perhaps if the Mummy had been more like the one played by Chris Lee in the Hammer 1959 version. Instead of Yvonne Furneaux, the Mummy's lost love could have been one of the KLS or Kate Jackson characters. It would have been more interesting if the Mummy had taken on the guise of a modern day antiquarian, rather like Im-Ho-Tep and Ardath Bey in the original 1932 film.
A complicated plot involving Laura, the resuscitated Mummy, a feline brother and sister AND Nicholas Blair hell-bent on bringing down Collinwood and getting his revenge, would have added a LOT of color to the otherwise dreary Summer of '71...
G.