It won't surprise me at all if Mauer signs with another team if, a) he feels another team with interest in him offers a better shot at getting him a WS ring, he believes he'd have a meaningful role on that team, and c) the money is good.
I don't believe sentiment will play a role, either on the part of the Twins FO offering him an opportunity to return OR on his part. Both parties will do what makes the most business sense for them.
He'll retire when he's no longer enjoying going to the ballpark.
I think many overestimate the importance, to him, of remaining with the Twins. I also suspect he's well aware of the disdain that a vocal portion of the Twins fan base holds him in. All in all, he won't hesitate for a moment to sign elsewhere if he feels it's in his best business interest to do so.
1.) What team offers PT and a better chance at a ring? Not a lot of contenders looking for a first baseman who doesn't have pop and is more of a defense/intangibles guy.
2.) Joe does have kids and is settled in MN/FL. That's why I think the only place he would consider going is Tampa. He has a house near there and his kids aren't in school yet. But is Tampa a better spot than the Twins? Assuming the Twins make an offer, it's hard to see Tampa being the team that trumps that or the Rays being a better contender than the Twins.
3.) Joe grew up a Twins fan and has a chance to be a lifelong Twin with his number retired, in the HOF and in the argument for best Twin of all time. I think that means more than you'd think. Plus his family and his wife's family is in the cities, it's hard to move away from that. Joe's a big family guy.
4.) The only thing I could see changing it would be if the new Twins management starts wanting to push him out. Joe's not going to take crap at this point. That could happen but I think there's not a strong reason for it to happen (assuming he doesn't fall off big time next year). He's not going to cost much and he has a skill the Twins need at a position (LH 1B) that they don't have other options (Rooker can play elsewhere, Sano can play DH if 3B isn't working, no big prospects coming, hard to see the Twins signing a free agent 1B). Plus, he still puts butts in seats and jerseys on backs. The intangibles of fan love are not going to bigger anywhere else than MN.
I think the front office won't want to piss off the larger section of the fan base that loves Joe, especially since there's likely not a good baseball reason to do so until 2021.