Presumably it's just going to be another (revisited) location, better graphics/animation/AI, stories are getting more complex, even bigger maps with even more side missions and discoveries, and probably another leap in terms in realism of NPC's -- much like RDR2 lets you have (elementary) conversations with virtually everyone, and NPC's follow much more realistic entire "paths" and activities during the day.
GTA has always been about unparalleled breadth, not depth. Deep and narrow is the DNA of other different types of games. And nobody has the resources to to deep and broad. GTA's breadth and shallowness is its defining feature, not its problem.