GTA IV tried to be too gritty and realistic.
I'm playing through San Andreas again at the moment and it's miles ahead of GTA4. They need a game with the scope of San Andreas again, airports, deserts, country sides, casinos, dams, rivers, gang territories, farms, connecting cities, free ways.
GTA4 compared to San Andreas is so ****. Saints Row 2 had the right idea, the mechanics just weren't robust enough.