1) Fully customizable hair colors with either RGB, HSB, or similar sliders, along with the preset colors.
2) More hairstyles. The choice in SR2 was nice, but some just weren't quite right.
3) Full-length camoflauge pants. All I could get in SR2 was shorts >.<
4) As many have suggested: more voices to chose from. There was a surprising variety in SR2, but there were a few glaring omissions: All of Asia went unaccounted for for the most part, and some good additions would include: Chinese, Japanese, Russian/Eastern European, Indian, Middle-Eastern, German, French, Posh Brit, Jamacian/Islander, Pacific Islander... the list goes on...
5) Iron out the glitches. While it was kind of amusing to take my bike up the side of the Phillips building... yeah.
Some ideas for new gangs, in generic terms:
A newer street-racer gang.
A pair of gangs that have an intense rivalry and are primaraly african-american, a-la Crips and Bloods.
A gang of comic-book villains, a-la batman (no superpowers, but funny outfits)
As I spend a LOT of time hunting hotdog-men... a gang of people in mascot outfits... might fall into the last gang listed, but *shrug*
A gang of preppy white people, who run protection rackets at the local mall and drugs at the university.