+Moritz SchubertWas thinking a limit of one Time Lord and 2 companions each (it could be a teamup between multiple doctors, so he is not unique across incarnations, in multiplayer) Doctor would also have a bonus (Time Lord) sphere much like Frodo/Bilbo in the Saga expansions. You could fit existing spheres (renamed) in and divide doctors amongst them. Think 5 doctors. Canon can be broken.
Although a Doctor Who version would need some tweaking, since each player could probably only control one hero (one plays the Doctor and the other his companion). On top of that, there would be a very restricted number of combinations of Doctors and companions since out of canon combos would probably feel pretty weird.