A recent post on GTAforums from insider Tez2 references datamined files within GTA 5 that seem to be borrowed from Rockstar's recent Western epic.
In their post (opens in new tab), Tez2 says that new strings had been found in the most recent update to GTA 5 that related "to the standalone version." One of those strings comes directly from Red Dead Redemption 2's version of Rockstar's RAGE engine, backing up claims from other forum users that the next-gen port of GTA 5 will also be built on that version of the engine. According to Tez, however, it's possible that only the updated UI will be based on the Red Dead Redemption 2 code.
Last June, Rockstar announced an "expanded and enhanced" PS5 port for Grand Theft Auto 5, which originally released in 2013 on the PS3 and Xbox 360, and has gone on to become one of the best-selling games of all time. There's been no word on an Xbox Series X port of the game yet, but Rockstar hasn't ruled it out.
The studio has been using its RAGE engine since 2006, using it to create GTA 4 and 5, both Red Dead Redemption games, and Max Payne 3. The engine has been updated over time, with a full rework ahead of the PS4 and Xbox One versions of GTA 5, and further refinement for Red Dead Redemption 2, which allowed for improved weather systems and more photorealistic graphics rendering.
It's not clear what other changes might be made ahead of the next-gen versions, but it's likely we'll hear more from Rockstar on that front closer to the port's release.
The next-gen port should help tide you over until GTA 6 comes around.