Destiny 2 Shadowkeep will mark the game's migration from Battle.net to Steam as well as a way to purchase the entire experience for the new players. It will also mark a significant increase in required disk space, going up to 165 GB.
Destiny 2 already requires 80 to 85 GB of free space on your disk, be it on a console or PC. For today's standards, it's kind of hefty but not a disaster. Other games, such as Red Dead Redemption 2 and Ghost Recon: Wildlands take up 100GB or even more space on our disk drives.
Bungie's flagship title will overshoot those, however, on 1 October 2019 when Shadowkeep expansion releases since the game will require a staggering 165 GB of free disk space. Console owners should start preparing for the massive requirement, while PC owners will likely have fewer woes, depending on whether they want Destiny 2 on HDD or SSD.
Steam store page currently lists the game requiring just 105 GB of free space but the newsletter Bungie are sending those who subscribed to it clearly states the requirement will be 165 GB "as of 10.1.19" or in other words, as of 1 October 2019.
Keep in mind that Bungie put up a disclaimer, saying it's possible the actual number might change leading up to the expansion's release date. There is no telling if it's for the better or worse.
Thankfully, the rest of system requirements shouldn't change drastically for PC since they are held back by consoles.
The themselves are not that high, with the recommended ones listing GTX 1060 6GB and AMD R9 390 GPUs, both of which are usually used in mid-range rigs. On the CPU front, i5 2400 or AMD Ryzen R5 1600X will suffice and both are fairly accessible, with decent price points.