Designing a home brew world? Simply ask the players "hey, what playable races do you want to see? What classes? How about the pantheon, if there is one?"
It feels like a lot of people make up campaign worlds as a hobby in the background, maybe while playing in other campaigns in many groups. So there might not be any people particularly to ask while it's being constructed. Or has been already running with a different group.
If it"s more spur of the moment, or for a particular group of players, then it does seem like getting their input would be a common thing.