Would be very hard to do. It's one thing to get the rights to a handful of legends, it's quite another to get the rights to entire teams full of classic players. Even NBA 2k always has some holdouts and basketball rosters are way smaller than football rosters.
I think a reasonable alternative would be a legends section of the roster. Depending on how many they get the rights to, they could have them either all in one place or have an all conference team for every conference. Not sure how that would work for Notre Dame legends, or how they would handle like USC or Oregon legends for example who played in the Pac 10 but those teams are now in the Big 10.
That's all semantics for what I want this for though. I mean I guess they could let people play quick play games with the all conference legend teams against each other or against the CPU. The main purpose of this at least for me would be the ability to import the legends onto any team in a roster file, or into a recruiting class in a dynasty, or directly onto a team in a dynasty.
Any legend they have the rights to (which means any player that shows up in ultimate team) should be available for something like this. I doubt it happens though because people have been asking for a form of playing with legends in franchise in Madden for years and they haven't done anything with it. I suspect it's because they want to keep the full list of legends that they have rights to under wraps so that they can build hype for certain ultimate team promos. Dynasty/Franchise players get screwed over again to cater the the UT whales.