Imagine spending years playing San Andreas. GTA IV hasn't been announced. Your mind races with the possibilities of a new game. You imagine they're going to do a whole country instead of a few cities. You think of all the mini games, all the weapons, and customization. It is the first "next gen" GTA after all.
Then you have been playing IV for years. GTA V hasn't been announced. Once again your mind races with possibilities. There is no end to the amount of detail it will have, there is nothing you can't do, there is no limit to the map.
Now you've been playing V for years. Here we go again. But there really isn't anything to imagine. You know it's going to be just a bigger map. At least until it gets close to release, where the devs admit the map is the same size as V. They claim it's okay because there's more things to do. But all that means is more things to collect, more so-so mini games.
You've played RDR2 so you know how much detail it could have. You know they'll simulate everything. So there can't be any real surprises. You'll see more wildlife. Your car windows will fog up. Your character will swat mosquitos.
You know the graphics can be just a little better, not like moving from PS2 to PS3. More reflections, slightly better lighting, a little more detail in the textures.
Now you realize you're on the plateau. There are no more great heights to surmount.There is only lateral movement from here on out. GTA VI can only be as good as V, with marginal gains in every category, but not exceptional gains in any. You'll drive from Point A to B. You'll shoot the gangs. You'll escort the cars. Everything you have already done.
There is a running theme in culture right now and it's that large, long-standing franchises are starting to eat shit. They can no longer push the needle. GTA will join them.
When you can no longer breed faster horses, you invent the car. There's many ways in which GTA can still make a leap to a new quality level. Imagine if you could destroy buildings and then see it being repaired for weeks before coming back to a functioning state. Imagine npcs like those of cities skylines 2, each with their own stats and needs and living their life in the simulation. Imagine a custom made AI to give life to those Npcs so they react to your actions and the news in the papers in a realistic way. Maybe even let you talk to them with your mic.
Imagine you go to a random npc house and kill the adults living there, leaving the teenager son alone in the world. Now he starts selling drugs to survive and that causes a new side quest to appear related to that drug-selling. Not something that was baked into the game but generated automatically based on your own actions.
Traffic too. Imagine traffic laws changing to react to the accidents you cause. Speed bumps showing up in streets with frequent crashes. People avoiding certain roads after frequent police chases. New roads being developed dynamically in the city in reaction to problems in the old ones.
Imagine you hiding in some bushes for your wanted level to go down and a few npcs are walking by and you hear them talking about the crimes you just committed, mentioning the number of victims and where the police had been chasing you.
Most of those ideas are within the means of an AAA budget game now.