Cassel is good at one thing, being a backup. Why would anyone sign him to be their starter?
In my opinion he is as good as gone next year. He has played exceptional and some team will give him the money he is looking for that the Pats won't. He knows he has an opportunity to be a starting qb and its not in NE. With the shortage on good QB's in the league something will happen and there is alot of examples in the past where backup qbs flourish and land a starting gig. See Scott Mitchell in Miami when he took over for Marino when he was injured.
There is prb better examples than that but I'm a Dolphins fan and thats the first thing that popped into head.
Trent Green has done it twice. He replaced Frerotte in 98' with the Skins and Warner in 01' with St. Louis. Matt Schaub is another example.While some(Rob Johnson and Mitchell to a lesser extent) haven't worked out, its not unheard of at all for one teams backup to become another teams starter and be productive.
Personally, I'd say its a 3-team race between New England, Detroit and Minnesota. I bring up Detroit and Minnesota for 2 reasons. First they both have issues at QB and second they both are likely to have new coaches next season(unless Minnesota sneaks into the playoffs.) There is a good chance Josh McDaniels is looked at for a head coaching job, especially by a team like Detroit who may be looking to snag some of that Patriots magic. New coaches like to bring in their own guys and it seems like a pretty natural fit to me.
Just a thought.