To add to what The Game said about uniform and logo makers -
I've owned every version of this game, and it's always been immersive to some degree. But the thing that brought this game to life for me, is FaceGen and the Uni/Logo flexibility.
That's not to say that all the other improvements to functions, stats, finances, etc etc haven't brought a lot of life to the game as well, but stats and game play were always in OOTP in some form.
When I could actually look at fictional players and give teams cool logos and unis was when immersion went to a whole new level.
I think all Mods add a lot to this game, but it's the ability to put a face to a name in a custom uniform that adds life.