Let's get these E3 2019 rumors starting off strong.

In 2011 a game was released wherein the main character could "hack" into
MindJack Watch Dogs 3

the minds of any other character and take control of them. The game was called MindJack (pictured above) and, in addition to the unique mind hacking, the game also featured a single-player story that could "transition seamlessly" into co-op. It wasn't all that well received.

Fast-forward to the present and a hot pre-E3 leak tells us that the idea behind MindJack may be very much alive and well in the year 2019. The third Watch Dogs title is apparently going to be called Watch Dogs Legion and, like MindJack, will allow you to take over any character in the game and play as them. I suppose you're taking over any "NPC" in the game, but calling them an "NPC" in this case doesn't seem very accurate since they, in fact, playable. So, they're just PC. Oh, that's going to ruffle a few feathers.

Whether or not any of these characters will be as full of (ahem) character as Marcus from Watch Dogs 2 remains to be seen. This all comes to light via TheNerdMag, a site that also accurately reported about Mortal Kombat 11 prior to the official reveal. I'm willing to bet this information is also accurate since a lot of it also comes from a leaked Amazon UK listing for the game that is still somehow publicly viewable. This is also "confirmed" by Jason Schreier.

According to the leaks, Watch Dogs Legion is set "in a near-future, dystopian version of London. It's a post-Brexit world in which society, politics and technology have changed and altered London's fortunes." That sounds like a very political plot to me, especially coming from a company that said just last month that they're "not trying to make political statements in (their) games" (Sebastien Le Prestre, lead dev at Ubisoft talking about Ghost Recon Breakpoint).

Anyway, continuing on with Watch Dogs Legion.

London is one of the most iconic cities in the world and has had a massive influence on all of Western Culture for centuries… London makes total sense for WD, as the city has one of the highest surveillance levels in the world making this the perfect playground.

Play as anyone, Every individual you meet in the open world, has a full set of animations, voice over, character traits and visuals that are generated & guided by gameplay systems.
Some have said the idea reminds them of MindJack while others have brought up the point about it sounding like Driver San Francisco, a game which was developed and published by none other than Ubisoft in 2011. Both games weren't all that great. I suppose we'll find out soon enough how Ubisoft is going to try to tackle this idea where you can play as generic NPC #1-1 million.