Well, the concept so far as it's been explained to my understanding is that this isn't the first Destiny game, it is the only Destiny (at least for a decade). Over the course of the next 10 years, through various expansion packs, we will explore, progress the story, and retake the Solar System, aquire new gear, multiplayer will evolve, etc. Basically they want to make sure the Destiny we're still playing in 2020, is the Destiny we're used to from 2014. I have a feeling there are still several planets (and locations on planets) that we've yet to see. Each planet is probably HUGE with tons of stuff to do, and so far we've only witnessed parts of one single mission per planet.