With both Sony and Microsoft having dropped hints that the next-gen will kick off in 2020, it makes sense that the big developers are gearing up for the new technology and hardware. Rockstar Games has posted several job openings related to next-gen developments, some of which hint at what the future may hold for Grand theft Auto.
Among a sweeping number of job openings throughout all of Rockstar's many offices around the globe, many descriptions hint at "next-generation worlds for some exciting upcoming projects". The use of next-generation out of context can be construed as merely being a buzzword to indicate high quality content, but considering we're a year away from the predicted beginning of the 9th console generation, it may be taken more literally.

Rockstar Games does things in very specific ways and over the years their style and tactics have become rather clear. They release few games, with large gaps in between launches, but the quality of those games are always stellar. They stick to a relatively small portfolio of hit franchises and rarely release multiple titles in the same franchise back to back, in order to avoid franchise fatigue.
Of their IP, GTA and Red Dead are the most popular and long-lived. Following the breakaway success of GTA 5, there was never any question about whether we'll see a sequel. With GTA 5 and Red Dead Redemption 2, we saw two radically different approaches to console generations.
GTA 5 was released right at the turnover of the 7th and 8th console gens. Developed with the Xbox 360 and PlayStation 3 in mind, the eventual re-release for the new platforms didn't make complete use of the new hardware — not that the ports were ugly in any sense. Red Dead Redemption 2, on the other hand, was developed for the Xbox One and PlayStation 4 from the get-go and released near the tail-end of this generation, making the most of the hardware.
One of the main tells that something is brewing on the GTA front is the job listing for a vehicle artist, whose tasks would include "sculpting and texturing a diverse range of vehicles of all types; including but not limited to Cars, Trucks & Vans, Motorbikes, Helicopters, Planes, Military, and Boats". That lineup of vehicles sounds very much like what we'd expect from a GTA title.