Antonio Rukavina has, according to reports, reached agreement on a contract extension with Villarreal. Ruka is a handy guy to have around, as he can play at both left back, right back, and even in midfield on occasion. I can't say he is playing as well consistently as he did for Valladolid, but then he is not seeing as much playing time either.
Apart from our various players on loan, he is the only player whose contract is expiring this year.
Now, as far as players who are under contract but attracting interest: big story last week was Pep Guardiola suggesting he could sign Eric Bailly for Manchester City. Don't know how serious that is, but Javi Mata spent the first few minutes of yesterday's match talking about it. Bailly's release clause is reportedly €40m.
Mateo Musacchio has been almost forgotten at times this season--he came back successfully from his ankle injury, but then has picked up muscle injuries (the latest on international duty with Argentina) and is not expected to return before season-end. His release clause is €50million, various EPL clubs have been rumored to be interested.
Mateo's contract expires in 2018 and he will be 26 in August (Bailly is 21, contract to 2020) so I would think the Argentinian is the more marketable and more likely to move of the two. Remember also Villarreal now owns all his rights, obtaining them from River Plate, so that would make a deal easier.
Finally, Cédric Bakambu is also attracting attention (and by the way, Happy Birthday, Cédric, 25 years young today!!). Supposedly EPL sides Swansea and Crystal Palace passed on him a year or two ago, and now they are wishing they hadn't. Bakambu's release clause is €50 million, reportedly.