recommend watching this video
however that vid just mainly about ‘rpg’ lvling games so here are my ideas
first you gotta sure what would appear in your game or what have to be appeared in your game
for an open world game, you think you must spend time to sketch up the ideas about building for players planning up their designs on ‘survival’ ways like cave, bridge [for camping for the players i guess] and ofc can’t skip the bushes - a great detail for player to hide.
area should be splited into many kinds, the starter area and fighting/ hunting/ camping etc… and some of your imagines which can holding player’s attraction to your game
after all, try to hold a meeting with your development, ask them for ideas and go on for a little design, mountains blablabla