I read an interview where Sam Houser, the guy in charge of Rockstar games, revealed that he wanted a fuel system in the games but got talked out of it by the developers. Maybe they prototyped the idea and found it more annoying than fun?
Possibly. I like to think that anything's possible, though, and that it's all in the way they approach things. For example, remember all those gas stations in San Andreas? Just scenery. But imagine if they were more than just scenery, and interactive, and Rockstar always strive to make their worlds more interactive. Fuel could be only active in free roam, unless a specific mission calls for it.