Can't imagine that happening and it would be pretty dumb by SF imo.
The thing with Brady is, that you don't really know how good he is anymore. He is just as much a product of the Belichick-/Patriots-system as the Patriots are a product of him. So I am not sure a) if he can be plug&play in every other team and b) if he is an upgrade to those teams that already have an above average QB (which Garoppolo certainly is).
With all his experience he has seen it all and is a fantastic game manager, but I don't think he is among the best anymore when it comes to be an executing QB. Drew Brees for example is a better QB than Brady just looking at the ability to distribute the ball through the air.
If he is leaving the Patriots, he would want a good paycheck, since he was playing on a pretty low wage all the time considering he is probably the GOAT. It's not like he needs the money, but he wants to be paid accordingly for the legend he is. So cap space is something to watch too. There is no point in getting Brady and then in consequence be forced to get rid of other good players - and thats not what Brady would want too.
He wants to play for a contender. For that contender, Brady must be a huge-ish upgrade at QB and they must have some cap space. Jacksonville comes to my mind, but according to
spotrac they are in cap hell. I am no expert when it comes to cap space issues, so not sure how much money they could save by cutting Foles. Tampa Bay has lots of cap space, a pretty good team overall and no QB.
Those rumors about SF getting rid of Garopollo seem to have appeared after the defeat in SB 54, and it does not really make a lot of sense to replace Garopollo with Brady when years ago NE wanted to replace Brady with Garopollo, even if Brady still has it you are only going to get 2 or 3 years out of him and then you will be back in the QB market looking for your next star, and as we know it can take you decades before you find your guy.
It is better that SF sticks with Garopollo and they stick as well to their run heavy scheme, after all it is working, they got to the SB, they only lost because in front of them was the monster that will dominate the NFL during the next decade, there are only a handful of QBs which could have won that game under those circumstances.
In the end I think he will just stay at the Patriots for this reasoning:
My guess is that the patriots pay him to stay, who else is patriots gonna get in place of him. Draft a high pick young qb and have him learn a year under the goat
That being said I have my doubts Brady will comeback to NE, we do not know how comfortable NE feels about Stidham but he is still too inexperienced and did not looked sharp during the few moments he participated during the regular season so I can see NE wanting Brady back, but does Brady really want to comeback to NE? While he wants to win I am sure his family is going to play a big factor this time around, which is why most of the teams he is speculated to go to are geographically close, SF, LA chargers and Las Vegas.