Not much is yet known about what we can expect to learn of Cruella's past, but Kelly Marcel (Saving Mr. Banks) is at work on the latest draft of the screenplay. She has taken over from Aline Brosh McKenna (Annie) in working out the trajectory that made Cruella the chain smoking hound hater we know and love.
Originating in the 1956 Dodie Smith novel that became Disney's animated classic, Cruella has been seen in live action twice already, portrayed by Glenn Close in both 101 Dalmatians (1996) and it's follow-up 102 Dalmatians (2000). Close is actually connected to this new film, but only as a producer so far.
Stone is currently "in talks", so there has been no signing on the dotted line as yet, although it is certainly sounding likely. There is no director in place yet either, but that should also be sorted soon, since Disney are hoping to get Cruella De Vil before the cameras this year.