According to that leaked contract info from back whenever (assuming it's still accurate/relevant, which I think part of it has changed already, so yeah), starting with Destiny 2, the series would not be available on last gen consoles, and would add PC to it's platforms. Unfortunately, I don't think they could add Destiny 2 onto 1 because of this, but it might be possible for 3 and 4 to add onto 2. As mentioned, this is all assuming that leaked info remains true. I hope it is, as my preferred platform is PC.