Interesting reading all the comments about who would like to get the second seat.
The biggest issue is contracts, it's great to talk about Alonso, Bottas, etc. but it their contracts are water tight they are not going to be able to move to another team. My gut feeling is Alonso is tied into McLaren and he couldn't move even if he wanted to no matter how much money is thrown at the situation. Also Honda want him badly as their main driver so I reckon they would block any move he tried to make. Personally I do feel that Fernando will not want to drive alongside Lewis again either.
Bottas is a slight possibility but again his contract is watertight and he's tied to the team for 2017.
The only possibility is a driver that is linked to the Mercedes brand so you could say the most obvious choice will be Pascal Wehrlein. He's ran in the car, and he knows the team.
You could even say Mick Shumacher is a possibility if he was a little older and had more experience. F1 drivers are getting into the sport younger and younger so whereas a few years ago I would have thought it crazy to have someone of Max's age in an F1 car it now seems almost the norm.
Whoever it is they will probably be given a one year contract so that leaves the door open for someone like Alonso or Max to try and make a run for the seat in 2018.
If Nico had announced this in the middle of the season it would have possibly allowed some top names to go for his seat (performance related contracts would come into play) but with the timing it makes it very tough for anyone to move from another team unless an agreement is made (in other words a lot of money changes hands!)
There could still be a surprise. I always remember reading that Eddie Irvine was going to be Schumacher's teammate in 1996, well I never saw that one coming!