The most likely is Vauclair. He was an AHL allstar this season and clearly deserves a shot. At the very least, he will get a long look in training camp and pre-season.
Redden, Chara, Phillips, de Vries, Volchenkov and Pothier will be back next year, but there is a need for a 7th d-man who remain with the big club full-time. My bet is it will be Vauclair, and he'd probably play 40+ games as the 7th man given the inevitability of injuries. One knee injury to a regular, and he's full-time.
Simpson and Leschyshyn are unrestricted free agents, and it is unlikely they will be back. It is much more likely other teams will be willing to give them a full-time job (and pay), rather than finish their careers as the 7th or 8th d-man in Ottawa.
Schubert also might have a shot, but Platil definitely needs more time in the minors.
Up front it will be quite crowded, although Charlie Stephens may have a shot to crack the line-up as a 4th liner. Payer also might have a shot, although he's a more likely call-up.
Mirnov, Kairgorodov, Eaves and Bochenksi will all get a shot to prove themselves at training camp (although none are guaranteed to be there), but any one of them would have to really knock the Sens brass socks off to stay with the big club. Think about how hard it was for Spezza to make it full time.
Emery will also get some consideration, but much of that depends on what happens with Lalime and Prusek, with much of that depending on what happens this post-season. If Lalime lays an egg in the playoffs and is the fall guy for an early round loss, it's quite possible he'd be moved, and Prusek and Emery would be the 1-2 next season.