Holy crap!!! Someone stole my idea. Of course, my idea is years old but nevertheless, my idea.
Basically, you want to load a roster from 2004 (final, opening, default, etc.) and play (or simulate) the 2004 season and then play the 2005 season as it was played in reality. You want to use the Draft Class to edit all the generic players into the actual players who were drafted in the 2005 NFL Draft (Alex Smith, Aaron Rodgers, Antre Rolle, etc.) and once you're able to get every player on their correct team, play the 2005 NFL season.
In short, you want to play a multi-season franchise but with 2005 coming as close as possible to what it was in reality without loading any other roster and/or franchise files.
If this is what you're talking about (which I think it is), let me know and I'll get back to you quickly.