if i could add a little something (im pretty sure none of these have spoilers):
-more pranks
-lots of more missions (all of them with unique objectives which rockstar is king of doing)
-more cliques, im thinking goth, popular (cheaper form of preppies, whatever u wanna call it), etc.
-even more complex combos
-swimming competitions (where u actually race other ppl)
-skateboard and bike tricks (i know its an action game but it would still be cool)
-a bigger yearbook (including teachers, landmarks, more students, etc.)
-completely customizable bikes and skateboards
-customized tags
im sure i can think of more but im tired right now