This is such a great idea. Car chases would certainly be more realistic, and since much of GTA revolves around car chases, this would certainly be a huge step forward for the series.
And the OP also mentioned the idea of blending into a crowd. That's an even better idea. If R* doesn't make LS a desolate wasteland with no people (this is something that every GTA thus far has struggled with) then this idea could be realistically pulled off. The severity of the crime committed could dictate how much attention one gets from pedestrians, which relates to how well one could truly blend into the crowd. For instance, beating up a guy would result in a few people noticing the crime, but sprinting away would ultimately allow one to escape barring any police presence in the area. Firing an RPG meanwhile, would result in people shouting, "That bastard in the red shirt is a terrorist!!" and everyone within the vicinity taking notice. Sprinting away wouldn't be enough to escape because the crowd would have already dispersed due to there being a "terrorist" in the area.