Well assuming he DOES want to give it a go, he'd have a number of hoops he'd have to go through in order to get to the position where he would become a viable candidate to come out of retirement.
1, He'd have to under go a 30-60 day intensive work out program to start with. I doubt very much he's currently close to his playing weight and strength right now.
2. He'd have to do what would amount to a "pro day" workout for the Pats, who probably have a record of what Marty did the last time he worked out for them, and have those numbers to be reasonably close.
3. He'd have to convince Bill that he really wants to make the tough mental commitment to play a full season in the Pats arduous system, and this isn't just a lark.
If Bill was happy with the results of all that, THEN they could discuss a the role he'd play, which, like Gronk, would be designed to get them both to the playoffs in relative good health and condition.