Yea, it's coming to PC. From what I'm told the decision to restructure was made thanks to a major memory ceiling being reached on last gen with regard to the Rise of Iron expansion, which was set to launch in the spring, not fall. Bungie decided to push out D2 since it was behind and needed much more iteration, and took advantage of the Rise of Iron delays to use that as the content stop gap for the fall of this year.

Destiny 2 is being overhauled so that they can build upon it in the future without having to work within the frame work limitations that D1 setup. Right now, the way things play out, they have to introduce playspaces so that players can engage in systems or activities, but it creates a huge barrier that must be overcome on both the networking & design side so, they're going to do more open playspaces that incorporate towns, outposts, etc into more common areas. This means fewer hardlines for areas and having them be gated by menus. Goal is to have players conducting things & making activity decisions while still in control of their character, instead of having to exit out into an activity hub. Think of how in WoW, you hit a button and queue into activities you want to do all while still playing your character.

Major narrative changes are on board for D2. Both in terms of plot direction, pacing & structure. Major focus is going to be on the Cabal & Saturn. Saturn is a playspace is set to be bigger than all of the playspaces currently implemented in the game combined. This is in large part of the scale of playspaces being redesigned, so it'll be both bigger & denser with activities. There's a reason Guardians take the fight to Saturn.

Major changes to the engine are occurring. Their render tech has gone through some major iteration thanks in large part to getting to approach it without having last-gen as a limit. Because of this, they are still undecided what, if any, of Destiny's old content, both in terms of playspace, areas, Strikes & raids, will be available through D2. There is talk in possibly allowing D1 remain as its own client, which flies against how Destiny currently operates, where as the new content renders the old client & content obsolete.

Live team has been going through some iteration so that they may continue to support Destiny with content after a major content drop, but they are stuck between a rock & a hard place. The live team is trying to support a game running on an old engine with its older tools, whereas the bulk of development is on the engine framework & enjoying new tools. Because of this, the Live team is largely handcuffed, so don't really expect a whole lot of post-launch content for D1 in the next year.

However, starting with the launch of D2, we will see far more substantial content dropping in the time between yearly releases, as the game & engine & live team have all been planned to support that going forward. Team is aware that they need more substantial content throughout the year, not just at the end of it.

If they do drop D1 content, it likely won't be permanent - they might reintroduce areas in a more 'D2' fleshed out way. So we might lose temporary access to these places, then regain it or have to fight back for control of it at a later point in the plot. This means we'll get to see older areas or activities through the perspective of the newer content, which also means the older areas might expand considerably by the time we get back to them, since the design goal for playspaces is to now make them both way bigger & way denser with activities.

Edit: New races & factions are planned. No idea on whats happening with regards to classes however. New enemies within factions are also going to be introduced. Take the Sions for the Cabal for example - they are going to get fleshed out in D2.