Mercedes will want to defend their constructors championship once again so will need another very good driver. With the new rules I don't think they will be satisfied with a secondary driver running full time. Look at Red Bull & Ferrari pair of drivers, I would say both better combined pairings then what Merc has currently with Wherlin so Merc line up is likely to change.
Alonso is my guess on who they will try to sign by breaking his contract with Mclaren. Toto has said previously if Nico doesn't resign Fernando would be his pick. http://www.planetf1.com/news/seven-p...s-for-rosberg/