I'll contribute with some ideas I have delevoped since 2000.
Dunkpacks
What I have always wanted is two different dunkpacks for every players. One for the breakaway and other for the in your face slam. For example Vince Carter would not make the same dunks on a breakaway as under the basket over 3 players (Live 2003 was a terrible example of this)
Behind the back passes
where have they vanished? 2001 had them and they were awesome
replay camera-angles
I don't like the replay camera-angles of Live 2003-2004 and 2005 I guess. 2001 was much better, but I would add one more camera-angle that would make my first suggestion perfect.... the camera-angle underneath the basket like in real-life dunk contests
player-specific moves
Live 2003 was supposed to have them but didn't. I would like to see something like MJ:s cup dunk, Dr J:s dunks, VC's elbowhang (oh wait, that wouldn't be that good in-game, but in practice?), AI's crossovers, shaq's monster jams and one handed easy dunks... you know what I mean. They would add a whole new level of realism
Facial expressions and closeups
2004 and 2005 are not consentrating on the little things that make the game sweet. Imagine if you got to the freethrow line with MJ, the closeup screen appears, while MJ is performing his freethrow ritual
chewing gum!
Freethrow rituals and shooting styles
yes
The basket
In the whole NBA Live series history, the rim has never ever been realistic to me. It always lacks detail and it always is too thin.
Pre-game presentations...
Are getting worse every year. NBA Live 2000 anyone? + ESPN's pregame presentation was jawdropping (at least what I saw on ign videos)
Pickup games
This is a feature I've always wanted. 1on1 yes, but 2on2, 3on3 and 5on5 on street court! Live 2001's street court has always been my favourite and I've always would have liked to play games on that

I would be very happy if someone made a mod for it...
anyhow, pickup games would be fantastic, even more if it would be more free and streetball-like
Training camps
Playing yourself instead of setting the sliders. I often find myself playing 20 mins of practice mode per playing session

so why not convert it to dynasty mode so you could improve your players' skills how you want?
Classic games and teams
I would die to play some classic NBA finals with classic teams, 80's Celtcis and Lakers, 90's Bulls...
Ability to save ingame
It isn't "icing on the cake" but it has to be mentioned. why is it so hard to add?
fps camera-angle
It could be fun if it was made right
Be-a-legend mode
Like in NBA Street. Imagine coming as a rookie, getting a shoe contract, negotiating deals, practicing to come better...
And the games you would be player-locked, but you couldn't control the other players as we are used to. Instead of that you would be forced to take your own role, and if you are the star player, you would get the rock.
If your career was succesful, you could be insterted to the Millenium All-Stars (which also should be added)
Sorry, I guess I went from icing on the cakes to the major suggestions EA could add

But these are one of the things I would really like to see!
edit.
I forgot one really important!
Bullet-time!!!
Straight from Max Payne, you could slow time down while either breaking your defenders ankles or in mid-air!!! Imagine! You could see what you should do in air when trying to avoid getting blocked! This could result in Dr J style swoops, which now could be controlled, not by just pressing the shoot button again. Then again, it would be cool to move the arm with the right analog stick and releasing the ball with the shoot button.
The bullet time would be handy when going for the dunk also

which reminds me of one suggestion more I forgot:
Total Dunk Control (or Freestyle Air 2)
Once in air, you could go into bullet time mode (or not) and while in the air, control your players body motion with the left analog stick and his arms with the right

Different button will result in a different slam (2 hands or one) but the right analog stick would be used for windmills or pumps etc
okay I can't think of anything more right now