This is such a hard topic. I have no idea what goes into a great collaboration, so I am going to pair an author that I have been reading for a long time with an author I discovered more recently.
Susan Meissner and Cathy Golhke
Their most recent books have both been WW2 fiction, but they can both write in any time period.
Angela Hunt and Tosca Lee
Both of these ladies tackle interesting and difficult historical characters. I would love to see a collab on Jezabel or some equally controversial person.
Lynn Austin and Siri Mitchell
I think these two would write a fun, light-hearted historical rom-com together.
C. E. Laureno and Sharon Hinck
I would love to see what kind of world these authors would come up with.
Lisa Wingate and Christa Parrish
I think these two would write a beautiful contemporary novel together.