I believe ultimate team would be be more enjoyable in the end run if each collection completed prompted the player with some sort of menu that gives you an option of multiple different legendary players to choose from to receive as the collection reward. Madden 12 is pretty bland because those who want to have the best teams all have the same players which I believe hurts the bong between the player and his team. It would also make it better for players to truly create their own personal "dream team" as many users choose to pay their hard earned money to complete collections it would be more enticing for the person to know they can pick the player they want on their team instead of picking a collection to complete.
This could also add to some interesting twists as you could even make the cards slightly different from one another making every users team feel truly unique when they play against an opponent since not everyone plays with the same set of players.
I believe it would be better in the long run of ultimate team because when one gets to the point in UT where they are completing collections other lesser players do not wish to play against you a majority of the time so differing reward cards would make grid iron battles of the ultimate team upper echelon more interesting as everyone's team won't be a carbon copy of one another.