An AT thru-hiker had this same question years back and invented the Packa.
I am not connected to this company in any way. I use one as I section hike the AT, and this year it has seen some intense rain and win. Works wonderfully. Serves as a pack cover if rain is just threatening but not yet arrived, you can leave the arms flapping and use the hood (which has a brim), wear the whole thing open, closed, half-zipped using two-way zipper, use the huge pit zips to help moderate heat. . . .