Rose and Donna are both cool.
What kind of plot is being considered? Will it be a simple matter of the Doctor dropping onto Scadrial and wreaking havoc in his characteristic way? Would there be a Scadrian villain, perhaps a nobleman? Might there be some sort of rift in the Cosmere that things like Sontarans or Cybermen are using to invade the Final Empire?
I'd meekly suggest running a lottery, then. Take an inventory of everyone who's interested in playing the Doctor, then draw their names at random to see who gets to go first. Then their name gets taken out of the bag for the next plot so someone else can get a turn, and so on and so on.