Final Fantasy 7 Remake: 10 Story Changes That Have Already Happened

In an interview with Famitsu Magazine, director Tetsuya Nomura described the Watchmen of Fate as such;

The watchmen of fate (the black mist in the trailer) are mysterious beings that appear before the party wherever they go. I hope that even those who have played the original game will enjoy the new elements.

These cloaked, mist-like figures appear to swarm the party at inopportune moments throughout the game and appear to be linked to the "bigger threat" that Aerith alludes to in the latest trailer. Whether this is Jenova or potentially even Meteor, it seems clear that the Watchmen of Fate are going to be a big part of the remake.

Although they do not appear in the original game, it's worth noting that they do share similarities with some unused concepts. In early versions of the game, the Sephiroth clones were depicted as floating, mist-like creatures.

Since Square Enix has a history of dusting off unused story elements, such as re-purposing Sorceress Edea for FF8, it's possible that this is where the inspiration came from.