Be realistic. Open World games of even AA grade are enormous effort, and costly. And to be honest. You have bigger chance to create open world game as indie dev than by working in AAA studio. As AAA studio won't take the risks to make AAA quality open world game. Yes indie open world game won't have awesome graphics, tons…
Game design is a senior position at virtually every company (unless you start your own). Generally you need to have a lot of experience before even being considered for a role in design that would mean you have any say on project wide design concepts. Environment art/level design is a good start on that path though.…