The last thing Nico needs is another Japanese teammate! Nico is still an unknown quantity, he needs to be stacked up against a known quantity. Michael Schumacher, Nick Heidfeld, Robert Kubica, even Heikki Kovalainen would give us some frame of reference.
If the Mercedes lacks competitiveness next season, all the negativity and blame could end up being heaped into his court!
For Nico, the ideal partner must be Nick Heidfeld as he's got plenty of experience and a regular points finisher while not being unbeatable by a team mate. I think they would make the ideal team together and certainly if Rosberg was to fail to shine the brighter of the two then people would start to ask a few questions.
While the rumours of Schueys inpending return continue to circulate it remains a "believe it when the lights go out and the cars leave the line" type of story. From Rosbergs point of view it would rob him of his place as defacto number one driver and take a lot of the limelight out of being with the team. While I'm sure he would relish beating Schuey there would always be the question of equality and support from Ross and Mercedes. I would be pretty sure that Rosberg would greet a Schuey signing with luke warm enthusiasm at best.
I hope Kobi makes it on to the grid next year but I don't think it will be with Merc. Great for him if it was though.
Can't be that unknown, I don't think Mercedes would risk their reputation on an unknown quantity as lead driver (nationality beside). I'm sure after 4 seasons in F1, we can probably agree that he is a capable driver but just hasn't had a winning machine under him.
If you look at the situation from Mercedes point of view I think you probably have to chose Heidfeld, although I think he has had a lack lustre F1 career.
From a fans point of view, Kobi, he gave more excitement in his short appearance then some drivers did all season.