Nasr is going to race in IndyCar next season, but he's still not given up on returning to F1 in the future, apparently. Can't see a seat opening for him any time soon though...
As for Senna, he's in WEC now, driving an LMP2 for Vaillante Rebellion team, so does Nelson Pique and Nico Prost too.
Quite a competition for that Williams seat now... Kvyat, Kubica, Di Resta, Wehrlein.