I am basing this on nothing but simple human psychology - there will be widespread disappointment at GTA6, no matter what it is, no matter how great it is, it doesn't matter. The wait has been a decade and a half long, people's expectations have built up. GTA6 will be an amazing game, and a great GTA game, but ultimately people will expect something to be a revelation for the entire industry (you can see this in this thread itself), and the final game will simply be... more GTA. Which is going to be great, but lead to LOADS of disappointed fans.