The strange thing about that is, got to mean almost certainly Van in at Mclaren, but Perez appears to have committed to FI so who takes the Williams seat? Wehrlein?
Wehrlein is looking impressive for that car but he's completely untested and Merc just gave a 2 year deal was it to Rosberg. So a year or two in Williams would work great for Wehrlein, see what he can do in a more competitive but not top car, Merc get to test him out before maybe making him the no.2 in a couple of years.
Otherwise I can't really see any big names wanting the second Williams seat, Perez could change his mind but nothing about the WIlliams team or setup suggests forward momentum while FI are currently beating them and have been improving at a far better rate.