It would destroy the point of the "heat". But here's my ending:
Pacino shoots De Niro but gets shot as well; Pacino lives, but is so weakened that he's unable to do anything but hold De Niro's hand - like the original ending. De Niro staggers out of the field, steals a car, and tries to get back to his girl. But the wound is fatal and his driving becomes quite erratic. The camera stays on De Niro's face for awhile, and once the car stops the camera pans out to reveal De Niro has driven all the way to the waterline on a beach. He dies staring out at the water on the beach