I dunno, I'd say that in terms of mechanics, Grand Theft Auto has evolved and changed quite a bit throughout its iterations. I see where you're coming from, but there was a massive jump from San Andreas to GTAIV, and similarly there was quite the jump from Vice City to San Andreas (With the RPG elements, swimming, etc). I mean, the only way they could really innovate more than they have been would be by making spin-offs or by going the Saints Row route and just going crazy, but GTA has always prided itself on a changing, detailed setting and realistic characters, so making an engaging story and a memorable environment is also a part of it. It's a hard concept to change without straying from what is GTA.