big gun, big bullet, modest velocity.
He would seriously consider a 10mm, I think. long slide commander in 10mm. He probably wouldn't go with a desert eagle. he never cared about not having enough ammo, he carried a full load and two speed loaders. he won't have a double stack anything.
I don't think he would change. the .44 special was exactly what he needed, and it was in the script. he's going to have to have magnum, it will have to have brand recognition among laymen, and it will have to be unique.
Keep in mind that back then, there wasn't a single cop in the force other than harry that carried a 29. You can be certain that now, just like back then, there would not be a single cop in his department using one. There probably aren't any actual street cops in america even now that carry a 29, but I may be wrong.
I think nothing will change, unless they just throw 40 years of harry history out the window. with the way hollywood works, he may be a glock fanboy.