GTA 6 will be a huge success because it's too big to fail. However, I think it will be underwhelming for many players. While the open world and immersive sim elements will be impressive, the missions will be rather linear and the sandbox will be quite ropey. Rockstar have failed to iterate on these elements in the recent titles. The game will also lose the clever satire and edgy humor that made the previous game's great. Rockstar have pretty much said that they're forgoing that in favor of being sensitive and PC.
GTA 6 will find an audience of fans, but it will leave many veteran GTA fans underwhelmed and alienated. In other words, expect Grand Theft Auto: Red Dead Redemption 2.