[SenorFunkyPants]

First choice of the available drivers must be Russell.
As a possible future replacement for Bottas or Hamilton he has to be top of the list if Williams will let Merc borrow him.

Otherwise its Vandoorne or Hulkenberg neither especially inspiring but at least Nico would have a chance to get that elusive podium.

EDIT: There is an advantage to Williams in letting Russell drive the Mercedes...in that his performance against Bottas
(and by extension Hamilton) might give them some idea of where they are with the Williams.
In the sense that... if he does well against Bottas then it indicates that any improvements in the Williams performance will come more from car development than because Russell is underperforming the car.