It seems that we are receiving a third entry in the Watch Dogs series as Amazon U.K. have listed Watch Dogs: Legion for Playstation 4. This was also confirmed by Jason Schreier at Kotaku who noted that he had heard from multiple sources within Ubisoft.
The description of Watch Dogs: Legion reads as follows:
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. 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.
Expect an official announcement at E3 2019.