Marcus Ericsson

Ericsson is the driver who has been in the shadows for the past four years in F1. After debuting for Caterham in 2014, he moved to Sauber in 2015 and scored points in some of the races. But soon after 2015, Sauber fell off the pace again and haven't been able to pose much of a challenge for points.
Like Palmer, Ericsson doesn't show moments of unbelievable brilliance but still puts on a solid show every time. He has been battling his teammate Wehrlein on track who is very highly rated by Mercedes, which speaks volumes of Ericsson's talents.
It has been reported by Autosport that Ericsson is considering more than one team for 2018, which means that he is in talks with Williams as they are the only team besides Toro Rosso and Sauber yet to announce their driver's line-up for 2018. Toro Rosso favour giving Red Bull Academy drivers a run in F1 which can mean Pierre Gasly (2016 GP2 Champion) being promoted to the pinnacle of Motorsports.
Thus, Ericsson is in with a chance of taking this much fancied Williams seat.