In 3.5 or maybe pathfinder there was a magic item called a Bow Staff, pretty basic item at a command word it could go from longbow to quarterstaff and back.
That was a spell in PF, at least, that let you use a bow as a quarterstaff.
Come to think of it, that might work alright for 5e. They could just base it off of Shillelagh, and go from there?