is it StarLancer? Sounds a little like the StarLancer I played on PC thousands of years ago and it was on Dreamcast. It involved alot of customization to your spacecraft, open world blasting, and difficult as fuck. It did have online multiplayer.
Basically you're a spaceship, it can be multiplayer and I think the world is infinite. I think you can level up, no part is blocked but if you go too far monsters will be much stronger. There probably were planets that served some purpose but I'm not sure.