In terms of being different from Little Mac, the Urban Champ would have Mac's strength, but even stronger, with a few drawbacks, such as:
1. NO super armor on ANY attacks, but as a result, due to fighting on the streets, his attacks are more aggressive than Little Mac, because he doesn't know how to hold back. The other problem is due to him having trouble controlling himself sometimes he may do 1-2% damage to himself. It'd be pretty uncommon for him to do this, though.
2. He would use weapons, such as crowbars, manholes, flower pots, and....well, whatever else you'd find/use in a street fight. I'd say switchblade, but I think that's taking it a little TOO far.
3. His special moves would all be different from Mac, with the exception of Mac's Rising Star up special and Slip Counter down special. As I've said before, the Urban Champ would be more aggressive than Mac, and due to this, his up special, would rise to a more crazy height, due to his hot-blooded nature and his Slip Counter would involve him grabbing his opponent after they've struck him during the Counter animation, resulting in him grabbing them and punching them back hard, and I mean HARD.
4. His standard special would not be the Straight Lunge, but the "Burst Barrage". With this, the Urban Champ would charge up, and where Mac's fist glowed red, the Urban Champ's entire body would turn red, with him getting more and more of a furious expression on his face. When the attack is fully charged, he will GO OFF, starting with a powerful punch, and kick to the ribs, ANOTHER kick to the ribs, a punch to the gut, a headbutt, and ending it with what could essentially be considered Mac's side smash attack. His side special would involve him tossing a random item from Urban Champ, like a flower pot, or even a manhole, among other things.
5. I imagine his redesign to be along the lines of having THREE outfits. One being his jean jacket with torn sleeves, while wearing a red-and-white striped shirt underneath, with jeans pants on. His other outfit would consist of wearing a baby blue t-shirt with a red long sleeve shirt at the bottom while wearing blue sweatpants. The last one would be...well...a simple white t-shirt, as a homage to Cody Travers of Final Fight/Street Fighter fame, and Axel Stone of Streets of Rage. Also, I guess if they want to be realistic, his default outfit would involve him having golden-blonde hair, as seen above. His alts. would involve blue hair, purple hair, and green hair. I'll think of the others later.
6. His FS would either be him and an army of Urban Fighters beating down on opponents, or the Urban Champ would basically use a grounded version of the Mii Brawler's FS.