I really hope Fernando doesn't go, as I'm fairly sure with his luck McLaren will built a monster of a car and Honda will finally get power parity with Mercedes just in time for him to jump into second place.
I'm also sort of hoping Mercedes do get a high end driver, Seb is unlikely as is anyone in Redbull overalls but I think Perez is actually the most likely candidate. We know he has the ability to dump his contract if he wants to as his sponsors were already prepared to buy him out of Force India if he wanted a Williams or Renault drive, which he himself turned down. Hulk and Grosjean are locked in and frankly neither are good enough but Perez I honestly thing is on par with Rosberg and could at least take it to Hamilton in the same hardware given he was able to podium twice in the far more humble Force India this year.
Button if he was willing (and McLaren let him go, as he's contracted for 17/18 to them even though he isn't driving) would be a superb option as he's been proven to work well with Lewis, but he seemed keen to go and is getting well up in age, not to mention was dipping in performance at the end of the year.
Kimi isn't going anywhere, and to be honest Wehrlein I don't rate as much as Ocon, he just didn't convincingly dominate Haryanto as he was expected to, seemed to have trouble matching Ocon (who Force India chose over him given a choice of both) and that crap he pulled with not switching the engine off in the gravel trap when told over and over is the sort of insubordinance that Mercedes has had an absolute gut full at this point out of Hamilton and wouldn't tolerate at all from a rookie.
Perez is IMO the only driver that's gettable who could keep Lewis honest and hold up his end of the constructors championship which is their priority one.
