If he ends up at 80, it could be something like this:
72nd at a slam
73rd, 74th, 75th at Masters - maybe Cincy, one of IW/Miami some time in the next two years, an indoors one like Paris or Shanghai
76th, 77th at Basel, and Basel or another random 500
78th, 79th, 80th at Halle, Doha and some other 250
Not a bad prediction. IW or Miami would be tough unless they speed up playing conditions there or something happens to Djokovic/Nadal.
In fact, that may be Mr. Federer's one chance to reach 100 single's titles and a few more slams. He would have to hope that he plays on without too much further decline, and that Nadal, Djokovic and possibly Murray don't make it for one reason or another. Their style of play is aging (in terms of mileage/injury) them faster than Mr. Federer. IF
those guys are out of the way I could see him winning majors and more titles, even #1 when he is 34, 35, given the current crop of younger men. Of course, if one or two of the younger men steps it up, then it would be extremely difficult.