Some ideas I had for districts (as well as some additional info on my fan-made factions).
Fort Reynolds: A huge military installation located north of Downtown. Very easy to gain notoriety here. Police are replaced by STAG and National Guard Troops. Key landmarks include the Delta Airfield, the Command Center and the Lakeside Memorial Bridge. Area is controlled by STAG
Lincoln Meadows: A massive agricultural area located south of Carver. The area is dotted with trailer parks, small towns, a nuke plant and several roadside diners. This district is based off of your typical small-town. Takes inspiration from cities such as Bakersfield. Sheriff Deputies and State Police replace regular police units in this district. This area is controlled by the New Age Cult (based off of the Muginiki, Boko Haram and Al Qaeda). The leaders of this group are philosopher turned terrorist leader Amed Saheal, ax-crazy psychopath Basharr Hassan and black-market weapon salesman George Carson.
Wilton: A large scale suburban district located south of Downtown and New Colvin. Home to the Steelport University, Wilton Shopping Center and Steelport Convention Center. Based off of more semi-urban cities such as Raleigh or Orlando. Controlled by Theta 13 (based off of groups such as MS13, Los Zetas and Gulf Cartel). Theta 13 is lead by ex-Federal Police captain Joseph Ramirez, deranged assassin Martin Vasquez and drug smuggler Alex Vargas.
Mt. Eden: A more urbanized version of Wilton. (Think Denver or Salt Lake City). The more low-level areas include an Ultor-owned research facility, a ski resort, an Olympic Stadium Complex and a software company named "Volatile" (nod to Volition). Controlled by Ultor.
Changes to existing districts.
New Colvin: District now sports a "Little Mexico" and "Chinatown". Safe-Word now resembles the Leadenhall Building from London. District is now controlled by the Monarchy (based off the Yamaguchi-gumi). The Monarchy is lead by Japanese business tycoon Yotsura Mikami, former Chinese Public Security Officer Olivia Wong, and pop-star turned nutjob Roi Kim.
Downtown: Has a wider variety of landmarks. (like what we saw in the concept art). Sierra Point has been converted into a military-themed museum. District is now controlled by the Tsars (based off the Bratva, Donetsk
Republic and IRA). The Tsars are lead by an ex-Spetsnaz operative turned gang-leader Vladimir Mikhali, former British politician Nigel Abbott and PR front-woman Maria Leonard.
Carver Island: Has gone through the most changes; Arapice Island has been reopened (although there are now police checkpoints around the Refinery and npc's wear protective gear), the casinos in Bridgeport now have a more modern less industrial eyesore look, and the Three Count Casino is now based off the International Commerce Center from Hong Kong. District is now controlled by the Archangels (based off the Hell's Angels with elements of PEGIDA and Golden Dawn). the Archangels are lead by a professional car thief called Patrick Rourke, an insane bigot known as James Steiner and a hardened gang veteran named Darrel Mason.
Stanfield: Technically Legal is now a high-rise hotel. Salender is now a cruise port with many of the old buildings replaced by shops, bars and "tourist traps". Nuke Plant has been replaced by a new building that resembles Address Hotel from Dubai. (With some inspiration from the Burj Al Arab Hotel). Controlled by the Liberators (based off of left-wing and right-wing militias such as the FARC, Black-Bloc, Black Panther Party, Oathkeepers and Modern Minute-Men). Leaders of the Liberators are a crazy left-wing SJW named Amelia Yui, a sleazy business man named Robert Doyle, and a right-wing conspiracy nut named Albert Johnson.