I think they need to shake up the gameplay mechanics tbh. As great as both GTA V and RDR2 were, the gameplay mechanics were beginning to show their age with how formulaic and rigid they were. It seems Rockstar has stuck to the same script in all their missions for years now - drive/ride to point A, kill/steal person/item X, run away or go to point B. Detract away from the script and it's an immediate mission fail.
I want to see them offer a bit more creativity and open-ended approaches to the objectives, and perhaps some impactful choices. I don't want or expect it to be a fully fledged RPG necessarily, but after playing BG3 I feel like some creative freedom would be very welcome.
NakeyJakey actually did an excellent video on this very topic years ago which sums up my sentiments perfectly: