First and foremost the new lady must be a suitable vessel that is she must be able to carry the mantle, have enough spiritual room. That is why Maggie might work and Faith probably not and Karen certainly not. That is why Lea's training of Molly probably helped, she learned to handle more and more power which might have stretched her spiritual capacity, or at least confirmed to Mab that she was a suitable spare vessel.
Changelings are probably perfect as vessel because they are already prepared for more power if they choose their fey side. It would be a logical move for Mab to make more children now to get some spare vessels for the coming BAT but it might be to late for that.
Molly will get an entourage and Mab's dead will probably be suddenly and unexpected. So the nearest suitable vessel and who is the nearest suitable vessel?
Lacuna!
Unless it is Sunday dinner with the parents.