It's not going to happen immediately. The Deron Williams sweepstakes have to play themselves out first, but it's starting to look like he'll remain a Net (via footbasket.com).
Assuming that happens, Nash looks like he'll be next on Mark Cuban's list.
As much as Nash wants to win, he's also shown in the past how much loyalty and familiarity mean to him. He had several chances to force a trade out of Phoenix but never actually pulled the trigger.
This puts Dallas in a good position. He's a former Maverick with ties to the Dallas community, so it's not like it would be a major adjustment for him. In addition, his best friend is none other than Dirk Nowitzki. Given their friendship and the fact that Dirk is a former Finals MVP, I can't imagine Nash wanting to play with anyone as much as he would Nowitzki.
In basketball terms, there isn't a better fit for Nash than Dallas. Unless Miami throws its hat into the ring, Dallas is the only true contender that will call him. At this point in his career I doubt Nash wants to be part of a mediocre team.
He already has chemistry with Nowitzki. Both have matured since their 2004 separation, but both are at points in their careers where they could really benefit from each other. Nash hasn't played with a scorer like Dirk since Amar'e Stoudemire left Phoenix, and Dirk hasn't played with a point guard like Nash since Jason Kidd realized he was old.
If Nash is willing to take a pay cut, that might allow the Mavericks to do some real damage in free agency. They enter the process with the goal of signing two maximum salary free agents, and if Nash is willing to leave some money on the table, they'll have plenty of money to chase other guys.
If they want another scorer, they can make an offer to Eric Gordon. If the Mavs want another big man, there are plenty out there. They can go for a few top tier players or a ton of depth.
Ultimately, Dallas is in a position to rebuild itself on the fly as a contender. Nash wants to win a championship, and the Mavs are only a year removed from having done just that.
They're a match made in free-agency heaven.