I think there's likely to be more character customization and depth in general (both on and offline). Hopefully they also take vehicle and weapon customization much further as well. I like to think the physics, world details and feedback will be a lot better as well; more like RDR2 (I hope), given how A-tier it was and everything felt. Finally, I suspect there might be better property customization or ownership, considering a big wishlist item for GTA 5 was always mansions and we never got it. (I can kind of understand why.)
I admit though, I am more apprehensive than excited. Maybe cause it's still a ways off, but the shitshow that GTAO has been for the last 1-2 years deeply worries me for GTA 6, especially it's online. I'm almost certain the BS will be dialed up to 11 with pricing or P2W features. Or they'll take a page from Ubisoft and Activision, call it AAAA to charge €80 and then still charge a premium for cosmetics like Modern Warfare's been doing.