Watch Dogs was quite quirky in the way that the open world worked. Everything more or less revolved around the player and most of the NPCs were pre-defined to be distrusting or just outright hostile towards you. It was fine for the missions and the storyline of the game, but it made trying to mod the game very difficult. There didn't seem to be a good way to get cops to be friendly towards you, nor was there much control of the wanted system or anything like that. Even if, for example, a criminal NPC in a side-mission fired a gun at you, you would still be 'blamed' for it by the game and the civilians would call the police against you.
That being said, there were massive changes in Watch Dogs 2 where they added a factions system. It was a huge improvement that the player could have a different 'heat' level with each faction and that they would also have their own 'felony' system as it's called in the game. Being able to be 'wanted' by the 580s for infringing on their turf, but still have other entities like the police or other gangs remaining neutral towards you is something that was done quite well.
Where WD2 really shone, though, was that it let the different factions enter into combat with each other and it did this using the same, or almost the same, system as the main wanted system for the player. I believe that the game actually started a new 'felony' on the hostile NPC and let the appropriate faction respond according to its configuration while also following the different 'stages' of the felony system where they'd either be actively chasing or hunting/searching. This meant that you could have the police cruising around the block, spotlights on, searching for a hostile NPC in basically the exact same way that they would for the player. I think there were potentially limits in terms of the number of pursuers and aggressors, probably so that things didn't get too crazy, but even so it might actually be the best implementation of a wanted system that I've seen in a game.
Sadly, the one thing that lets Watch Dogs down from a modding perspective is that they are very different to GTA as far as the scripting is concerned. In GTA, scripts control almost everything in the game. In WD, the scripts are far more mission-centric and rely more heavily on the game engine. In practical terms, this means it is more difficult to do things like spawning, playing animations, controlling NPCs, and so on. Most of the things that you can script in WD relate to the player and not the world, or other cars, or other NPCs like in GTA.
In fact, to fully appreciate WD2 for what it is, these two videos do a great job in explaining how the AI works, and how the open world shines.
https://www.youtube.com/watch?v=HTK-bVRzp5k
https://youtu.be/W9KNH6X3nj4?t=127