This screenshot isn't new but it is revealing. Kurt Warner last played in the NFL in the 2009-2010 season. Of the 24 players on Madden NFL 13's "Canton Greats" roster of throwback stars, he was the last to retire. And here we see his return to the league touted in the game's new "Connected Careers" mode.
This is interesting for a couple of reasons. "Connected Careers" will allow you to play as any active coach or player. Playing as a coach means you control an entire team, like the old Franchise mode; playing as a player allows you to play as a single player, like Superstar. Everyone works in the same universe, with some limitations. For this, there are also some Hall of Fame coaches and players that are available for use in Connected Careers. But the assumption is that they will only show up in your league if you (or if another human in an online-enabled league) chooses to play as them.