Rockstar Games did amazingly well with GTA4. It really distinguished itself from its competition Saints Row so I think GTA5 is going to be just a little crazier than The Ballad of Gay Tony with planes, parachutes, and such but I don't think it's going to reach the San Andreas level of wackiness.
I'm rank 10 on GTA4 and I still play it today. I don't understand the dissatisfaction of the game and this is coming from a guy who's favorite GTA is Vice City. The story of GTA4 was very serious and I loved that. I found it a lot better because it didn't have wacky missions like in GTA:SA where you had to steal a jetpack from the military for some hippie. I believe GTA5 is going to be as realistic as possible while including things like planes and parachutes to please the fans because they don't want to get into the same field as Saints Row.
If you really liked San Andreas then just get Saints Row 3. The game is extremely wacky, fun, and unrealistic because I think you won't be satisfied with GTA5 if you hated GTA4. Grand Theft Auto is my favorite video game and I really hope they continue the series in a more serious tone.
I own:
GTA4
GTA:SA
GTA:VC
GTA 3
GTA: Vice City Stories
GTA: Liberty City Stories
GTA: The Episodes of Liberty City
GTA 2
and I have played all the rest except GTA London, even GTA advanced (piece of shit) so this opinion is coming from a veteran of the series.